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.
Implement vxl parser.
Implement hva parser.
Store the voxel normals table in a palette.
Add core voxel rendering code.
Allow mods and maps to define voxel sequences.
Integrate voxels with world renderer / render traits.
Voxel test mod.
Mirror sideways