I'm surprised that there's not some kind of real time-ish callback when the screen is about to update, giving the game a last chance to update its buffer? Or b

I'm surprised that there's not some kind of real time-ish callback when the screen is about to update, giving the game a last chance to update its buffer? Or better yet there should be a way of saying "use this buffer next time you update the screen" (and don't block during vblank time).