Use wlroots scene graph

[inclement]

Complements https://github.com/inclement/vivarium/pull/130. (Hopefully) fixes the rendering issues exposed in vivarium by the latest wlroots by switching to the scene_graph api instead of using our own damage tracking.

This is a relatively big change, but the scene_graph api is fairly straightforward and vivarium doesn't try to do anything special that needs extra treatment so hopefully isn't a big issue.