[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)
..and if you do that, then you could also easily allow `size=1` for making cubes, which might obviate the need to have two classes at all..