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

It's not valid because it depends on scenegraph; this kind of behavior is only allowed for VisualNode subclasses.
You could try using the method described in my visual tutorial 5, but perhaps we should wait until that PR is settled..