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

[storm]

As Thomas Dinges already answered, yes, it is very experimental modified
Cycles, you better stay of if you not coder and interested in that
bidirectional sampling. It have so many bugs and produce wrong pictures
except very small subset then only i am can make, as i know patch
limitations. Actually, with that last patch I already have found very
wrong shades when number of bounces more then 1. When it will be
finished, you expect to get something like
Luxrender/Mitshuba/Yafaray/etc bidirectional mode. The reason i post
that unfinished patch is i found that geom factor used was wrong, only
one cosine term was applied (correct formula is (cos(a)*cos(b)/t*t) ,
was cos(b)/t*t ), and after i fixed that i get much more pleasent result
then before, that is all.