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

src/thread/SDL_systhread.h

@@ -25,7 +25,6 @@
 #ifndef SDL_systhread_h_
 #define SDL_systhread_h_
 
-#include "SDL_thread.h"
 #include "SDL_thread_c.h"
 
 /* Set up for C function definitions, even when using C++ */

src/thread/SDL_thread.c

@@ -22,10 +22,8 @@
 
 /* System independent thread management routines for SDL */
 
-#include "SDL_thread.h"
 #include "SDL_thread_c.h"
 #include "SDL_systhread.h"
-#include "SDL_hints.h"
 #include "../SDL_error_c.h"
 
 

src/thread/SDL_thread_c.h

@@ -23,7 +23,6 @@
 #ifndef SDL_thread_c_h_
 #define SDL_thread_c_h_
 
-#include "SDL_thread.h"
 
 /* Need the definitions of SYS_ThreadHandle */
 #if SDL_THREADS_DISABLED

src/thread/generic/SDL_syscond.c

@@ -26,7 +26,6 @@
    implementation, written by Christopher Tate and Owen Smith.  Thanks!
  */
 
-#include "SDL_thread.h"
 
 #include "../generic/SDL_syscond_c.h"
 

src/thread/generic/SDL_syscond_c.h

@@ -20,7 +20,6 @@
 */
 #include "../../SDL_internal.h"
 
-#include "SDL_thread.h"
 
 #ifndef SDL_syscond_generic_h_
 #define SDL_syscond_generic_h_

src/thread/generic/SDL_sysmutex.c

@@ -22,7 +22,6 @@
 
 /* An implementation of mutexes using semaphores */
 
-#include "SDL_thread.h"
 #include "SDL_systhread_c.h"
 
 

src/thread/generic/SDL_syssem.c

@@ -22,8 +22,6 @@
 
 /* An implementation of semaphores using mutexes and condition variables */
 
-#include "SDL_timer.h"
-#include "SDL_thread.h"
 #include "SDL_systhread_c.h"
 
 

src/thread/generic/SDL_systhread.c

@@ -22,7 +22,6 @@
 
 /* Thread management routines for SDL */
 
-#include "SDL_thread.h"
 #include "../SDL_systhread.h"
 
 #ifdef SDL_PASSED_BEGINTHREAD_ENDTHREAD

src/thread/n3ds/SDL_sysmutex_c.h

@@ -25,7 +25,6 @@
 
 #include <3ds.h>
 
-#include "SDL_mutex.h"
 
 struct SDL_mutex
 {

src/thread/n3ds/SDL_syssem.c

@@ -26,7 +26,6 @@
 
 #include <3ds.h>
 
-#include "SDL_thread.h"
 
 struct SDL_semaphore
 {

src/thread/ngage/SDL_sysmutex.cpp

@@ -24,7 +24,6 @@
 
 #include <e32std.h>
 
-#include "SDL_thread.h"
 #include "SDL_systhread_c.h"
 
 struct SDL_mutex

src/thread/ngage/SDL_syssem.cpp

@@ -24,8 +24,6 @@
 
 #include <e32std.h>
 
-#include "SDL_error.h"
-#include "SDL_thread.h"
 
 #define SDL_MUTEX_TIMEOUT -2
 

src/thread/ngage/SDL_systhread.cpp

@@ -28,8 +28,6 @@
 
 extern "C" {
 #undef NULL
-#include "SDL_error.h"
-#include "SDL_thread.h"
 #include "../SDL_systhread.h"
 #include "../SDL_thread_c.h"
 };

src/thread/ps2/SDL_syssem.c

@@ -28,8 +28,6 @@
 #include <stdlib.h>
 #include <timer_alarm.h>
 
-#include "SDL_error.h"
-#include "SDL_thread.h"
 
 #include <kernel.h>
 

src/thread/ps2/SDL_systhread.c

@@ -27,8 +27,6 @@
 #include <stdio.h>
 #include <stdlib.h>
 
-#include "SDL_error.h"
-#include "SDL_thread.h"
 #include "../SDL_systhread.h"
 #include "../SDL_thread_c.h"
 #include <kernel.h>

src/thread/psp/SDL_syscond.c

@@ -28,7 +28,6 @@
    implementation, written by Christopher Tate and Owen Smith.  Thanks!
  */
 
-#include "SDL_thread.h"
 
 struct SDL_cond
 {

src/thread/psp/SDL_sysmutex.c

@@ -24,7 +24,6 @@
 
 /* An implementation of mutexes using semaphores */
 
-#include "SDL_thread.h"
 #include "SDL_systhread_c.h"
 
 #include <pspthreadman.h>

src/thread/psp/SDL_syssem.c

@@ -27,8 +27,6 @@
 #include <stdio.h>
 #include <stdlib.h>
 
-#include "SDL_error.h"
-#include "SDL_thread.h"
 
 #include <pspthreadman.h>
 #include <pspkerror.h>

src/thread/psp/SDL_systhread.c

@@ -27,8 +27,6 @@
 #include <stdio.h>
 #include <stdlib.h>
 
-#include "SDL_error.h"
-#include "SDL_thread.h"
 #include "../SDL_systhread.h"
 #include "../SDL_thread_c.h"
 #include <pspkerneltypes.h>

src/thread/pthread/SDL_syscond.c

@@ -26,7 +26,6 @@
 #include <errno.h>
 #include <pthread.h>
 
-#include "SDL_thread.h"
 #include "SDL_sysmutex_c.h"
 
 struct SDL_cond

src/thread/pthread/SDL_sysmutex.c

@@ -23,7 +23,6 @@
 #include <errno.h>
 #include <pthread.h>
 
-#include "SDL_thread.h"
 
 #if !SDL_THREAD_PTHREAD_RECURSIVE_MUTEX && \
     !SDL_THREAD_PTHREAD_RECURSIVE_MUTEX_NP

src/thread/pthread/SDL_syssem.c

@@ -26,8 +26,6 @@
 #include <sys/time.h>
 #include <time.h>
 
-#include "SDL_thread.h"
-#include "SDL_timer.h"
 
 /* Wrapper around POSIX 1003.1b semaphores */
 

src/thread/pthread/SDL_systhread.c

@@ -20,8 +20,6 @@
 */
 #include "../../SDL_internal.h"
 
-#include "SDL_system.h"
-#include "SDL_hints.h"
 
 #include <pthread.h>
 
@@ -48,7 +46,6 @@
 #endif
 #endif
 
-#include "SDL_thread.h"
 #include "../SDL_thread_c.h"
 #include "../SDL_systhread.h"
 #ifdef __ANDROID__

src/thread/pthread/SDL_systls.c

@@ -19,7 +19,6 @@
   3. This notice may not be removed or altered from any source distribution.
 */
 #include "../../SDL_internal.h"
-#include "SDL_thread.h"
 #include "../SDL_systhread.h"
 #include "../SDL_thread_c.h"
 

src/thread/stdcpp/SDL_syscond.cpp

@@ -21,7 +21,6 @@
 #include "../../SDL_internal.h"
 
 extern "C" {
-#include "SDL_thread.h"
 }
 
 #include <chrono>

src/thread/stdcpp/SDL_sysmutex.cpp

@@ -21,7 +21,6 @@
 #include "../../SDL_internal.h"
 
 extern "C" {
-#include "SDL_thread.h"
 #include "SDL_systhread_c.h"
 }
 

src/thread/stdcpp/SDL_systhread.cpp

@@ -23,7 +23,6 @@
 /* Thread management routines for SDL */
 
 extern "C" {
-#include "SDL_thread.h"
 #include "../SDL_thread_c.h"
 #include "../SDL_systhread.h"
 }

src/thread/vita/SDL_syscond.c

@@ -28,7 +28,6 @@
    implementation, written by Christopher Tate and Owen Smith.  Thanks!
  */
 
-#include "SDL_thread.h"
 
 struct SDL_cond
 {

src/thread/vita/SDL_sysmutex.c

@@ -22,7 +22,6 @@
 
 #if SDL_THREAD_VITA
 
-#include "SDL_thread.h"
 #include "SDL_systhread_c.h"
 
 #include <psp2/kernel/threadmgr.h>

src/thread/vita/SDL_syssem.c

@@ -27,8 +27,6 @@
 #include <stdio.h>
 #include <stdlib.h>
 
-#include "SDL_error.h"
-#include "SDL_thread.h"
 
 #include <psp2/types.h>
 #include <psp2/kernel/error.h>

src/thread/vita/SDL_systhread.c

@@ -27,8 +27,6 @@
 #include <stdio.h>
 #include <stdlib.h>
 
-#include "SDL_error.h"
-#include "SDL_thread.h"
 #include "../SDL_systhread.h"
 #include "../SDL_thread_c.h"
 #include <psp2/types.h>

src/thread/windows/SDL_syscond_cv.c

@@ -20,8 +20,6 @@
 */
 #include "../../SDL_internal.h"
 
-#include "SDL_hints.h"
-#include "SDL_thread.h"
 
 #include "../generic/SDL_syscond_c.h"
 #include "SDL_sysmutex_c.h"

src/thread/windows/SDL_sysmutex.c

@@ -31,7 +31,6 @@
  */
 
 
-#include "SDL_hints.h"
 
 #include "SDL_sysmutex_c.h"
 

src/thread/windows/SDL_sysmutex_c.h

@@ -22,7 +22,6 @@
 
 #include "../../core/windows/SDL_windows.h"
 
-#include "SDL_mutex.h"
 
 typedef SDL_mutex * (*pfnSDL_CreateMutex)(void);
 typedef int (*pfnSDL_LockMutex)(SDL_mutex *);

src/thread/windows/SDL_syssem.c

@@ -35,9 +35,6 @@
 
 #include "../../core/windows/SDL_windows.h"
 
-#include "SDL_hints.h"
-#include "SDL_thread.h"
-#include "SDL_timer.h"
 
 typedef SDL_sem * (*pfnSDL_CreateSemaphore)(Uint32);
 typedef void (*pfnSDL_DestroySemaphore)(SDL_sem *);

src/thread/windows/SDL_systhread.c

@@ -24,8 +24,6 @@
 
 /* Win32 thread management routines for SDL */
 
-#include "SDL_hints.h"
-#include "SDL_thread.h"
 #include "../SDL_thread_c.h"
 #include "../SDL_systhread.h"
 #include "SDL_systhread_c.h"

src/thread/windows/SDL_systls.c

@@ -25,7 +25,6 @@
 
 #include "../../core/windows/SDL_windows.h"
 
-#include "SDL_thread.h"
 #include "../SDL_thread_c.h"
 
 #if WINAPI_FAMILY_WINRT