[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)
One problem is this operation is done *after* the translation, which means that it will likely be out in the middle of nowhere :) Want me to open a PR into your branch that puts it into a somewhat-usable state, or make a bunch of comments here?