Improve vertex formats

[David Williams]

PolyVox currently provides two vertex formats (PositionMaterial and PositionNormalMaterial) and this should be more flexible. In particular:

1. Vertex formats could store the positions as integers/chars where this makes sense (e.g. with the CubicSurfaceExtractor).
2. More control is needed over which properties of a voxel get copied into the vertex data, and an appropriate format should be used. Differentiate between renderable and non-renderable properties, so that colour, material id, light intensity, etc could be copied across while health and physical properties would not be.

Also consider that if a mesh is being generated for physics purposes (i.e. a collision mesh) then we may want the physical properties but not the renderable ones.

We already have some relevant work in the cubiquity-version branch, but there is still a lot of work required to do this properly.

[David Williams]

A lot of improvements have been made here (see CubicVertex and MarchingCubesVertex) but it is still not as flexible as the system described above. Further work would therefore still be interesting.