Move to MatrixStack and RenderLayer-based rendering

[zml2008]

Minecraft seems to be trying to move away from immediate-mode rendering -- parts of the game have been converted already, others are yet to be converted.

While we can continue using the old way of rendering for now, I expect new major versions of the game to make that harder and harder, so it's probably a good idea to explore switching the CUI over whenever someone has time, rather than doing a rushed conversion right before a MC release.