* Fixed an old problem where if both the camera and a solid surface were
inside a volume, the volume wouldn't attenuate in front of the surface (was
visible in the blender conference art festival clouds animation:
http://mke3.net/blender/devel/rendering/volumetrics/vol_shade_cam_inside.mov
* Initial support for refracting solids inside volumes. I might be able to
make this work better, but not sure at the moment. It's a bit dodgy,
limited by the code that does the recursive ray shading - it's really not
set up for this kind of thing and could use a refactor very much.
834fda50d9