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

@lcampagn this is the code to get the transform from camera to scene, required to calculate the view direction vector in the vertex shader.