Clean up thread local storage when quitting SDL

SDL_Quit() should be the last SDL API that you call before exiting your application, and is intended to clean up all internal state.

If real-life applications are relying on thread-local storage after SDL_Quit() we could potentially add a hint to control this behavior.

Fixes https://github.com/libsdl-org/SDL/issues/6200

src/SDL.c

@@ -505,6 +505,8 @@ SDL_Quit(void)
      */
     SDL_memset( SDL_SubsystemRefCount, 0x0, sizeof(SDL_SubsystemRefCount) );
 
+    SDL_TLSCleanup();
+
     SDL_bInMainQuit = SDL_FALSE;
 }