Implement SDL_main as header-only lib for Win32

(remaining platforms will follow)

SDL_main.h is *not* included by SDL.h anymore, users are supposed to
include it directly now, usually only in the file they implement main() in.
If they need the header elsewhere or don't want SDL_main to implement
main() (but only call SDL_SetMainReady() or whatever), they
can #define SDL_MAIN_HANDLED first, same as before.
For SDL-internal usage, I added _SDL_MAIN_NOIMPL, which *also* skips the
implementation and `#define main SDL_main`, but still defines
SDL_MAIN_AVAILABLE and SDL_MAIN_NEEDED in SDL_main.h, as before.

To make the implementaion in the header shorter and avoid including windows.h,
I moved most of the Win32 SDL_main code into SDL3.dll via SDL_Win32RunApp(),
so the header-only part is just the different main functions calling
SDL_Win32RunApp(SDL_main, NULL)

Note that I changed changed the return value and type of OutOfMemory()
to return -1 instead of FALSE, so main() (or WinMain() or whatever)
returns -1 instead of 0 in case of an out-of-memory error

Compared to original Win32 SDL_main, I tweaked the part of the
implementation in SDL_main_impl.h a bit to avoid linker warnings
and conflicts with stuff from windows.h:

- replaced windows.h with own define of WINAPI
  and typedef-ing HINSTANCE and LPSTR.
  This prevents conflicts between all the generically-named #defines and
  types in windows.h and user code (like DrawState in some SDL tests)
- only using one of main() or wmain() gets rid of a MSVC linker error
  ("warning LNK4067: ambiguous entry point")
  If this still causes problems, we might try getting rid of wmain(),
  seemed to me like MSVC can use regular main() in UNICODE mode as well
- simplified the UNICODE logic for that - while this is not exactly
  equivalent to the old, it should make sense and Works For Me

src/core/windows/SDL_windows.c

@@ -331,6 +331,78 @@ void WIN_RectToRECT(const SDL_Rect *sdlrect, RECT *winrect)
     winrect->bottom = sdlrect->y + sdlrect->h - 1;
 }
 
+/* SDL_Win32RunApp(), which does most of the SDL_main work for Win32 */
+#ifdef __WIN32__
+
+#include <shellapi.h> /* CommandLineToArgvW() */
+
+/* Pop up an out of memory message, returns to Windows */
+static int
+OutOfMemory(void)
+{
+    SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR, "Fatal Error", "Out of memory - aborting", NULL);
+    return -1;
+}
+
+DECLSPEC int
+SDL_Win32RunApp(SDL_main_func mainFunction, void * xamlBackgroundPanel)
+{
+
+    /* Gets the arguments with GetCommandLine, converts them to argc and argv
+       and calls SDL_main */
+
+    LPWSTR *argvw;
+    char **argv;
+    int i, argc, result;
+
+    argvw = CommandLineToArgvW(GetCommandLineW(), &argc);
+    if (argvw == NULL) {
+        return OutOfMemory();
+    }
+
+    /* Note that we need to be careful about how we allocate/free memory here.
+     * If the application calls SDL_SetMemoryFunctions(), we can't rely on
+     * SDL_free() to use the same allocator after SDL_main() returns.
+     */
+
+    /* Parse it into argv and argc */
+    argv = (char **)HeapAlloc(GetProcessHeap(), HEAP_ZERO_MEMORY, (argc + 1) * sizeof(*argv));
+    if (argv == NULL) {
+        return OutOfMemory();
+    }
+    for (i = 0; i < argc; ++i) {
+        DWORD len;
+        char *arg = WIN_StringToUTF8W(argvw[i]);
+        if (arg == NULL) {
+            return OutOfMemory();
+        }
+        len = (DWORD)SDL_strlen(arg);
+        argv[i] = (char *)HeapAlloc(GetProcessHeap(), HEAP_ZERO_MEMORY, len + 1);
+        if (!argv[i]) {
+            return OutOfMemory();
+        }
+        SDL_memcpy(argv[i], arg, len);
+        SDL_free(arg);
+    }
+    argv[i] = NULL;
+    LocalFree(argvw);
+
+    SDL_SetMainReady();
+
+    /* Run the application main() code */
+    result = mainFunction(argc, argv);
+
+    /* Free argv, to avoid memory leak */
+    for (i = 0; i < argc; ++i) {
+        HeapFree(GetProcessHeap(), 0, argv[i]);
+    }
+    HeapFree(GetProcessHeap(), 0, argv);
+
+    return result;
+}
+
+#endif /* __WIN32__ */
+
 #endif /* defined(__WIN32__) || defined(__WINRT__) || defined(__GDK__) */
 
 /*