The parameter to SDL_AppEvent() should be non-const

This allows functions like SDL_ConvertEventToRenderCoordinates() to work without having to copy the event.

Fixes https://github.com/libsdl-org/SDL/issues/10691

test/loopwave.c

@@ -120,7 +120,7 @@ SDL_AppResult SDL_AppInit(void **appstate, int argc, char *argv[])
     return SDL_APP_CONTINUE;
 }
 
-SDL_AppResult SDL_AppEvent(void *appstate, const SDL_Event *event)
+SDL_AppResult SDL_AppEvent(void *appstate, SDL_Event *event)
 {
     return (event->type == SDL_EVENT_QUIT) ? SDL_APP_SUCCESS : SDL_APP_CONTINUE;
 }