Switch header convention from #include "SDL.h" to #include <SDL3/SDLh>

I ran this script in the include directory:
```sh
sed -i '' -e 's,#include "\(SDL.*\)",#include <SDL3/\1>,' *.h
```

I ran this script in the src directory:
```sh
for i in ../include/SDL3/SDL*.h
do hdr=$(basename $i)
   if [ x"$(echo $hdr | egrep 'SDL_main|SDL_name|SDL_test|SDL_syswm|SDL_opengl|SDL_egl|SDL_vulkan')" != x ]; then
        find . -type f -exec sed -i '' -e 's,#include "\('$hdr'\)",#include <SDL3/\1>,' {} \;
    else
        find . -type f -exec sed -i '' -e '/#include "'$hdr'"/d' {} \;
    fi
done
```

Fixes https://github.com/libsdl-org/SDL/issues/6575

include/SDL3/SDL_main.h

@@ -0,0 +1,273 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2022 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+
+#ifndef SDL_main_h_
+#define SDL_main_h_
+
+#include <SDL3/SDL_stdinc.h>
+
+/**
+ *  \file SDL_main.h
+ *
+ *  Redefine main() on some platforms so that it is called by SDL.
+ */
+
+#ifndef SDL_MAIN_HANDLED
+#if defined(__WIN32__)
+/* On Windows SDL provides WinMain(), which parses the command line and passes
+   the arguments to your main function.
+
+   If you provide your own WinMain(), you may define SDL_MAIN_HANDLED
+ */
+#define SDL_MAIN_AVAILABLE
+
+#elif defined(__WINRT__)
+/* On WinRT, SDL provides a main function that initializes CoreApplication,
+   creating an instance of IFrameworkView in the process.
+
+   Please note that #include'ing SDL_main.h is not enough to get a main()
+   function working.  In non-XAML apps, the file,
+   src/main/winrt/SDL_WinRT_main_NonXAML.cpp, or a copy of it, must be compiled
+   into the app itself.  In XAML apps, the function, SDL_WinRTRunApp must be
+   called, with a pointer to the Direct3D-hosted XAML control passed in.
+*/
+#define SDL_MAIN_NEEDED
+
+#elif defined(__GDK__)
+/* On GDK, SDL provides a main function that initializes the game runtime.
+
+   Please note that #include'ing SDL_main.h is not enough to get a main()
+   function working. You must either link against SDL3main or, if not possible,
+   call the SDL_GDKRunApp function from your entry point.
+*/
+#define SDL_MAIN_NEEDED
+
+#elif defined(__IOS__)
+/* On iOS SDL provides a main function that creates an application delegate
+   and starts the iOS application run loop.
+
+   If you link with SDL dynamically on iOS, the main function can't be in a
+   shared library, so you need to link with libSDLmain.a, which includes a
+   stub main function that calls into the shared library to start execution.
+
+   See src/video/uikit/SDL_uikitappdelegate.m for more details.
+ */
+#define SDL_MAIN_NEEDED
+
+#elif defined(__ANDROID__)
+/* On Android SDL provides a Java class in SDLActivity.java that is the
+   main activity entry point.
+
+   See docs/README-android.md for more details on extending that class.
+ */
+#define SDL_MAIN_NEEDED
+
+/* We need to export SDL_main so it can be launched from Java */
+#define SDLMAIN_DECLSPEC    DECLSPEC
+
+#elif defined(__PSP__)
+/* On PSP SDL provides a main function that sets the module info,
+   activates the GPU and starts the thread required to be able to exit
+   the software.
+
+   If you provide this yourself, you may define SDL_MAIN_HANDLED
+ */
+#define SDL_MAIN_AVAILABLE
+
+#elif defined(__PS2__)
+#define SDL_MAIN_AVAILABLE
+
+#define SDL_PS2_SKIP_IOP_RESET() \
+   void reset_IOP(); \
+   void reset_IOP() {}
+
+#elif defined(__3DS__)
+/*
+  On N3DS, SDL provides a main function that sets up the screens
+  and storage.
+
+  If you provide this yourself, you may define SDL_MAIN_HANDLED
+*/
+#define SDL_MAIN_AVAILABLE
+
+#endif
+#endif /* SDL_MAIN_HANDLED */
+
+#ifndef SDLMAIN_DECLSPEC
+#define SDLMAIN_DECLSPEC
+#endif
+
+/**
+ *  \file SDL_main.h
+ *
+ *  The application's main() function must be called with C linkage,
+ *  and should be declared like this:
+ *  \code
+ *  #ifdef __cplusplus
+ *  extern "C"
+ *  #endif
+ *  int main(int argc, char *argv[])
+ *  {
+ *  }
+ *  \endcode
+ */
+
+#if defined(SDL_MAIN_NEEDED) || defined(SDL_MAIN_AVAILABLE)
+#define main    SDL_main
+#endif
+
+#include <SDL3/begin_code.h>
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ *  The prototype for the application's main() function
+ */
+typedef int (*SDL_main_func)(int argc, char *argv[]);
+extern SDLMAIN_DECLSPEC int SDL_main(int argc, char *argv[]);
+
+
+/**
+ * Circumvent failure of SDL_Init() when not using SDL_main() as an entry
+ * point.
+ *
+ * This function is defined in SDL_main.h, along with the preprocessor rule to
+ * redefine main() as SDL_main(). Thus to ensure that your main() function
+ * will not be changed it is necessary to define SDL_MAIN_HANDLED before
+ * including SDL.h.
+ *
+ * \since This function is available since SDL 3.0.0.
+ *
+ * \sa SDL_Init
+ */
+extern DECLSPEC void SDLCALL SDL_SetMainReady(void);
+
+#if defined(__WIN32__) || defined(__GDK__)
+
+/**
+ * Register a win32 window class for SDL's use.
+ *
+ * This can be called to set the application window class at startup. It is
+ * safe to call this multiple times, as long as every call is eventually
+ * paired with a call to SDL_UnregisterApp, but a second registration attempt
+ * while a previous registration is still active will be ignored, other than
+ * to increment a counter.
+ *
+ * Most applications do not need to, and should not, call this directly; SDL
+ * will call it when initializing the video subsystem.
+ *
+ * \param name the window class name, in UTF-8 encoding. If NULL, SDL
+ *             currently uses "SDL_app" but this isn't guaranteed.
+ * \param style the value to use in WNDCLASSEX::style. If `name` is NULL, SDL
+ *              currently uses `(CS_BYTEALIGNCLIENT | CS_OWNDC)` regardless of
+ *              what is specified here.
+ * \param hInst the HINSTANCE to use in WNDCLASSEX::hInstance. If zero, SDL
+ *              will use `GetModuleHandle(NULL)` instead.
+ * \returns 0 on success, -1 on error. SDL_GetError() may have details.
+ *
+ * \since This function is available since SDL 3.0.0.
+ */
+extern DECLSPEC int SDLCALL SDL_RegisterApp(const char *name, Uint32 style, void *hInst);
+
+/**
+ * Deregister the win32 window class from an SDL_RegisterApp call.
+ *
+ * This can be called to undo the effects of SDL_RegisterApp.
+ *
+ * Most applications do not need to, and should not, call this directly; SDL
+ * will call it when deinitializing the video subsystem.
+ *
+ * It is safe to call this multiple times, as long as every call is eventually
+ * paired with a prior call to SDL_RegisterApp. The window class will only be
+ * deregistered when the registration counter in SDL_RegisterApp decrements to
+ * zero through calls to this function.
+ *
+ * \since This function is available since SDL 3.0.0.
+ */
+extern DECLSPEC void SDLCALL SDL_UnregisterApp(void);
+
+#endif /* defined(__WIN32__) || defined(__GDK__) */
+
+
+#ifdef __WINRT__
+
+/**
+ * Initialize and launch an SDL/WinRT application.
+ *
+ * \param mainFunction the SDL app's C-style main(), an SDL_main_func
+ * \param reserved reserved for future use; should be NULL
+ * \returns 0 on success or -1 on failure; call SDL_GetError() to retrieve
+ *          more information on the failure.
+ *
+ * \since This function is available since SDL 2.0.3.
+ */
+extern DECLSPEC int SDLCALL SDL_WinRTRunApp(SDL_main_func mainFunction, void * reserved);
+
+#endif /* __WINRT__ */
+
+#if defined(__IOS__)
+
+/**
+ * Initializes and launches an SDL application.
+ *
+ * \param argc The argc parameter from the application's main() function
+ * \param argv The argv parameter from the application's main() function
+ * \param mainFunction The SDL app's C-style main(), an SDL_main_func
+ * \return the return value from mainFunction
+ *
+ * \since This function is available since SDL 3.0.0.
+ */
+extern DECLSPEC int SDLCALL SDL_UIKitRunApp(int argc, char *argv[], SDL_main_func mainFunction);
+
+#endif /* __IOS__ */
+
+#ifdef __GDK__
+
+/**
+ * Initialize and launch an SDL GDK application.
+ *
+ * \param mainFunction the SDL app's C-style main(), an SDL_main_func
+ * \param reserved reserved for future use; should be NULL
+ * \returns 0 on success or -1 on failure; call SDL_GetError() to retrieve
+ *          more information on the failure.
+ *
+ * \since This function is available since SDL 3.0.0.
+ */
+extern DECLSPEC int SDLCALL SDL_GDKRunApp(SDL_main_func mainFunction, void *reserved);
+
+/**
+ * Callback from the application to let the suspend continue.
+ *
+ * \since This function is available since SDL 3.0.0.
+ */
+extern DECLSPEC void SDLCALL SDL_GDKSuspendComplete(void);
+
+#endif /* __GDK__ */
+
+#ifdef __cplusplus
+}
+#endif
+#include <SDL3/close_code.h>
+
+#endif /* SDL_main_h_ */
+
+/* vi: set ts=4 sw=4 expandtab: */