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

Sure, this needs to be implemented in a better way. One idea is to let the transformsystem object have a "cameratransform" (or something with a better name). On the event object this transform will be implemented in the way that I did it here.