Currently supported passes:
* Combined, Z, Normal, Object Index, Material Index, Emission, Environment,
Diffuse/Glossy/Transmission x Direct/Indirect/Color
Not supported yet:
* UV, Vector, Mist
Only enabled for CPU devices at the moment, will do GPU tweaks tommorrow,
also for environment importance sampling.
Documentation:
http://wiki.blender.org/index.php/Doc:2.6/Manual/Render/Cycles/Passes
