Blender Git Loki

* Volume Rendering

Finally in 2.5 branch :)

Still things to do, but will continue working in here. I won't bother
repeating the commit messages from the last year or so, however I've
written up some technical docs to help Ton/Brecht/etc review and find
their way around the code:
http://wiki.blender.org/index.php/User:Broken/VolumeRenderingDev

That above page has some known issues and todos listed, but I'm still
interested in bug reports.

Credits for this code:
* Matt Ebb
(with thanks to Red Cartel/ProMotion Studios)
* Raul Fernandez Hernandez (Farsthary) for patches:
o Light cache based multiple scattering approximation
o Initial voxeldata texture code
o Depth Cutoff threshold
* Andre Susano Pinto for BVH range lookup addition
* Trilinear interpolation adapted from pbrt
* Tricubic interpolation from libtricubic