[Bf-cycles] Volumetric patch rebased

[storm]

TL;DR: Another boring update, nothing to see here.

I have no significant news related to volumetric or bidir, but see many
changes in trunk, so that is quick rebase.

I rethink Density input parameter, now it directly modify extinction
coefficient, sigma = density * density_geometry_factor. No more fancy
-log(1-density) trickery. Need to fix description and defaults, maybe
later.

Trying to respect multiple scattering transition color, so Homogeneous
mode is broken, only g=0 and constant color make sense, but when it work
it produce images that i think right. Need more tests.

Bi-dir part not changed. Transparent and other BSDF with delta function
have wrong weight. But in some cases it work better then previous in
"spot light rays in fog street" scenes, due fix in homogeneous volume
weight calculation.

I am too lazy to support Bi-dir and vcolume integrator for new
non-progressive case, I think better fix bugs and finish at least one
mode then spread.
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