Hello I have made some tests

The 3D skybox: It works perfectly but we loose the depth sensation if the camera doesn't move.
As I understand now objects are at an unreachable location. It's possible to have a 1/16 displacement for the camera ?

God rays: I tested them and they look better if they aren't 100% white. Maybe we should add an option to modify the color per track.
http://pix.toile-libre.org/?img=1379244018.png (quick and dirty hardcoded modification in the shader).

I found a strange bug with alpha-testing grass. Sometime a "dark" artifact appear.

Thanks :).