Cycles: remove disabled kernel_volume_get_final_homogeneous_extinction code.

I'm not quite sure why this would be needed. It gives per color channel sigma,
which we should have so we can compute per channel attentuation without noise
instead of the single float now.

However I think this should be handled at a deeper level by making sigma a
float3 everywhere instead of doing some kind of correction for this afterwards.

Cycles Volume: remove some unnecessary rngs from shadow computation.

Cycles Volume: add appropriate shader contexts for OSL.

This is needed to avoid using same memory block twice when the closures from
a shader evaluation are in use at the same time as the closures from another
evaluation.

* New Volume Absorption node was missing in Node Editor menu.

Cycles Volume: implement volume absorption node.

This is the transparent volume node renamed, currently it is basically the
same as a scatter volume node with anisotropy 1, so it scatters light perfectly
forward.

This also does some tweaks to the henyey-greenstein closure code to avoid
division by zero and to make eval/sample consistent in some corner cases, and
some other code cleanup related to volume shaders.

Merge branch 'master' into soc-2013-dingto

Cycles Volume: materials without a surface shader now act transparent when
there is a volume shader.

This is the behavior you would expect I think and makes setup easier. More
optimizations are possible in this case to avoid surface shading altogether, but
it's probably not so noticeable altogether as this will early out and not get to
any expensive computations as far as I know.

Note that these still count as transparent bounces even if there is no BSDF. Not
entirely happy with that, but there should be some way to limit the depth?

Cycles Volume: remove use volumetrics option.

In general it should not be needed to turn on such features, cycles should
auto-detect if there is a volume shader and just use it. It would be useful
at some point to have global options to turn off effects, but we better group
those options somewhere then.

Cycles Volume: rename isotropic to scatter volume, g to anisotropy, scattering to volume bounces.

Merge branch 'master' into soc-2013-dingto

Merge branch 'master' into soc-2013-dingto

Merge branch 'master' into soc-2013-dingto

Solved conflicts with __device to ccl_device change, was actually quite easy.

Merged revision(s) 60839-61042 from trunk/blender into soc-2013-dingto.

Merged revision(s) 60651-60838 from trunk/blender into soc-2013-dingto.

Merged revision(s) 60459-60650 from trunk/blender into soc-2013-dingto.

Volume:
* Style and code cleanup for volume.h closure file.
* Revert rename of henyey-greenstein closure to isotropic, the actual node name is still unsure, but the closure name should be called what it is.

Volume:
* Remove the "start" variable from the sampler code.
This was hard coded to 0.0f and therefore had no effect on the calculations.

Volume:
* More style and code cleanup.

Volume:
* Some renaming and move common epsilons to #defines.

Volume:
* Some code cleanup for shadow_blocked_new, renamed to shadow_blocked_volume() now.
* Also re-order the parameters, so the first 4 match the shadow_blocked() function.