[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)
It *should* be faster to initialize this with a set of vertices and a set of indices, as opposed to a long list of vertices. Don't forget to try to use `create_cube` instead of using manual indices, too.