Blender Git Loki

* Added a button to the volume material controls 'Alpha' to generate an
alpha channel based on the volume's transmission properties, allowing you
to use it in comp etc.

I'd rather not have this button at all, and make it just work properly
by default, however it causes problems with overlapping volumes when
'premul' is on (stoopid thing..) so for the time being, there's the
button. I'll try and fix this up later on when I have more time.