Draw events!

[aseemk]

Just as it's helpful to know when animation has begun, is happening and has ended, it can be really useful for apps to know when drawing has begun, is happening and has ended. The last one in particular is equivalent to "the image has resolved (for now at least)", which can be really useful for e.g. taking a snapshot, knowing that it's okay to move (e.g. slideshow), hiding a throbber, etc.

We could implement this similar to how we implement the animation events: each frame, see if the drawer needs an update; if it does, it's waiting for new tiles, which means it's still drawing; if it doesn't, and animation has finished, drawing is finished. See this Get Satisfaction post where I share this idea.