Add renderer support for matrix uniforms.
Add renderer support for additional vec* uniforms.
Expose depth buffer to render code.
Expose stencil buffer to render code.

The intention is to provide a layer for tracking shadow
rendering, so a single bit is sufficient for now.
Support Vertex objects with custom z coordinate.
Store normal vectors in the palette texture.
Implement vxl parser.
Implement hva parser.
WIP: Add voxel renderer.

 - Only supports the Gl renderer (needs Cg shaders).
 - The VoxelSheetBuilder mess needs replacing with something sane.
Add sequences equivalent for voxels.
Add a widget for UI voxels (e.g. colorpicker).
Initial implementation of RenderVoxel* traits.
WIP: Shadows