[WIP] 3D Visuals

This pull request implements basic 3D visuals, like a 3D polyhedron or a 3D cube. I think it is a prerequisite for completing volume rendering as a visual. (https://github.com/vispy/vispy/pull/458)
If you want to keep these, then I think it is necessary to document the order of vertexes / faces (or at least link to the docs for `create_cube`, which should do the same).