[Bf-cycles] Volumetric patch update

[storm]

Two major very old bugs fixed, one that i use wrong dimension for first
random vector for paths starting from light, as result distribution of
"pure bidir" chains was completely wrong. ost noticeable was different
brightness close to  different sides of emissive plain in fog. Need more
time to cleanup band-aids that i was use to mitigate that in past, but
at least now it produce pictures that i cannot find visual artifacts,
despite known unimplemented features as proper asymmetric BSDF support
(wrong smooth normals and bumps), wrong flat mirror/transparent pdf,
many other, cannot remember all.

Second bug was related to volume sampling distance pdf. I completely
reconsider and rewrite it, dropping my old (really stupid) ideas, now
use proper absorption/scattering coefficients triades. User visible is
new "Volume BSDF NEW" node. In case HDRI sky as only light source in
scene it can render almost all possible scenes, even glass filled with
media (real glass hull, with IOR differ from 1.0). So, i get milk in
glass scene render, it not perfect, but all previous attempts was
completely bad in comparison. It still in rewriting, so many things not
work. I temporary turned off "Density geom factor", it always 1 now, one
must use absorption and scattering for that. Homogeneous seems work as i
expect, Woodcock delta tracking need more love, i have trouble with
properly handle scattering pdf, so no cool "hard volume shapes", at lest
they will look wrong.

Also, patched Cycles in bidirectional mode now can render not only in
preview, but in final mode, even animation. Thanks Brecht who make patch
to force tiles stay in one big memory chunk, you need "Progressive
refinement" on to get it.

I think after some cleaning, can go to final stage, add proper MCMC/MLT
initial paths seed pool and use it as start points, that will make
somewhat that can compete to Luxrender default MLT mode. Theory is easy,
and we already have triangle light distribution code that can be reused,
just need some coding. If it will succeed, i hope more ppl will look at
patch and enhance it, as working MLT sure have perfect "WOW" factor to
attract ppl, as some scenes will get huge speedup . Will see.

Do NOT save important scenes with patched version, it can lead to broken
scene file that you will can not open later in next version of patch and
completely lost data. Always keep original. 
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