[Bf-cycles] Volumetric patch update (now with new shiny bidirectional integrator)

[storm]

Few fixes all over patch, nothing really important. Trying to make milk
scenes with transparent hull, need to hack deeper how direct light and
"transparent shadows" work, move MIS from direct_emission to later, as
we do not know yet from trnasparent shadow loop is it have some
intersect (not finished).

Must note, two recent trunk fixes (BVH corner case related to multi
threading, and BSDF NaN fix for extrsme angles) make significant boost
on few my test scenes, reducing time (BVH quality) and remove most
fireflies, not i get feel like use some commercial render here ^^.

About spectral GSoC, very naive implementation is very easy, as in my
patch you can try it already, but physically correct, that utilise fact
that RGB eye stimulus curves, use importance sampling for that, and
accurate handle in every BSDF is hard enough. Even good correct
dispersion is not linear (i use hard coded simple linear).

Unfortunately it will increase pressure on GPU resources, at least 1
more dimension to sample, one extra shader member, more registers, and
cannot be just added to main Cycles module. Maybe make separate module,
and choose old one if we have 0 spectral related materials. I think same
for my patch, better do it transparent for user, pre-process scene
before every frame and if it have 0 bidir light, spectral materials,
volume shings go good old Cycles path.
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