Expose a wrapper around SDL_QuitMainCallbacks to allow early exit when using callbacks

[playmer]

Someone ran into the issue of needing to early exit deep into the call stack when using callbacks and asked what the proper way to do so would be.

I suggested something like the following:

SDL_AppQuit(appStateTheUserMightNotHave, SDL_APP_FAILURE);
SDL_Quit();
exit(1);

And after poking around it seems we have SDL_QuitMainCallbacks internally which seems superficially similar to this, albeit without the call to exit. Can we expose something that wraps this and calls exit? I think the only immediate concern is that some platforms (I'm looking at iOS, with it's call to SDL_UpdateLifecycleObserver as an example) might need to do some cleanup, so that might need to be abstracted into a function for this wrapper to call.

Does something like this sound reasonable?