Initial work on Pica rendering.

Adds basic pipeline stages for the Pica GPU:
* vertex loading
* vertex shader
* primitive assembly
* clipping
* rasterization

For most of these things, only the bare minimum to run the gpu2 demo is implemented.

TODOs:
- [x] address review comments
- [x] fix Windows build
- [x] get the pending PRs that this branch relies on merged.
- [x] fix a minor regression in the rasterizer.