Blender Git Loki

* Volume rendering - z transparency

This solves one of the last remaining hurdles for
volume rendering. Previously it always used ray
tracing to shade other objects inside or behind the
volume. This meant that said objects would look
aliased, unless you used Full OSA on the volume
(which is slow!). As well as this, it meant that you didn't
get a good alpha channel out of the volume to use for
compositing, similar to ray refracting materials.

This commit enables z transparency for volume
materials. Although it can be potentially less
physically correct, in most situations there's no
difference, and you get the benefit of nice sampling for
other objects and an alpha channel for compositing too.