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/testpen.c

@@ -107,7 +107,7 @@ static Pen *FindPen(SDL_PenID which)
     return NULL;
 }
 
-SDL_AppResult SDL_AppEvent(void *appstate, const SDL_Event *event)
+SDL_AppResult SDL_AppEvent(void *appstate, SDL_Event *event)
 {
     Pen *pen = NULL;