Blender Git Loki

Volume:
Cleanup and refactor of closure code in volume.h.
* Remove unused code (the brute force henyey-greenstein eval and sample function and double peaked functions). Maybe we need something here in the future, but the current code just became too confusing.

* Remove the CLOSURE_BSDF_DOUBLE_PEAKED_HENYEY_GREENSTEIN_ID, instead use CLOSURE_VOLUME_ISOTROPIC_ID now.
The final naming of the node/closure is still unsure, but better to use one consistent naming scheme. When the "g" parameter is 0, the closure is Isotropic anyway, so maybe it's ok, we'll see.

* Now we use 2 counterparts to bsdf_eval() and bsdf_sample():
volume_eval_phase() and volume_sample()

ToDo:
* Reflect these changes for OSL.