WIP: Volume rendering

This PR implements volume rendering. It's basically a port of the technique that I use in Visvis.

Todo:

- [ ] make volume available in demo-data
- [ ] make camera information nicely available in vertex shaders, also for e.g. imposters
- [ ] implement other rendering techniques

Indeed:

https://github.com/vispy/demo-data/blob/master/brain/mri.npz

And it's selfishly an example I want to see look nice :)