Nirvana rendering postprocessing

Add renderer class to issue the drawing steps.
Render water particles in separate drawing step.
Use scene depth buffer in manipulator to position the hand.

This doesn't work anymore on Intel (HD3000), as this driver seems to have a problem with framebufferobjects without color a attachment. I think it's okay for us to work on (newer) Nvidia/AMD, (or newer intel?) ?

(ref #40 related)