Blender Git Loki

* Raytraced shadow casting for volumes

This is a first version and still has a couple of things undefined or
unimplemented, such as external objects casting shadows on or within volumes,
however volume->solid shadows are going ok.

http://mke3.net/blender/devel/rendering/volumetrics/shadows_test_02.mov
http://mke3.net/blender/devel/rendering/volumetrics/vol_test_shad3.blend

As with other transparent raytraced shadows in Blender ,in order to make it work,
you must enable 'TraShad' on the material *receiving* the shadow. It would be
nice to make this a bit easier to use, since there's not much chance you want a
volume material to be casting solid shadows, but that's a bigger issue in the
renderer outside this scope.

The volume shadows are working from the same physical basis of absorption, and
support coloured absorption:

http://mke3.net/blender/devel/rendering/volumetrics/vol_shad_absorption.png

They also work properly with multi-sampled (i.e. QMC) soft shadows:

http://mke3.net/blender/devel/rendering/volumetrics/vol_shad_sharp.png
http://mke3.net/blender/devel/rendering/volumetrics/vol_shad_soft.png

And by popular request the test file:
http://mke3.net/blender/devel/rendering/volumetrics/vol_test_shad_clouds.blend