Nirvana terrain

Adds basic terrain rendering and interaction, equivalent to the old osg implementation.

Interaction works with Alt pressed + keys R/F for up/down and mouse move to pull the grabbed terrain.
Interacting position is the (not visible) hand position.

Does not load the rock texture, we should add this once we are able to load resources (objects, textures.. the hand).

We should improve the rendering techniques when your are done with your CG exercises =)