[Bf-cycles] Volumetric patch update

[storm]

Markov Chain Monte Carlo (MCMC) a.k.a Metropolis Light Transport (MLT).

No BS, it works. Ugliest ever hack to utilise Brecht tile based thread
scheduler/manager in very perverted way, need to be rewritten ASAP, but
it can render pictures. Expect wrong overall brightness, i think i
calculate it wrong.

If you dare to compile it and even run, try Mutations = 10000, it
usually work safe.

Brecht, i tested it a bit, and think it worth further development, as in
some scenes it really rocks (method of sampling, _not_ my ugly patch).

To all: it still slower then Cycles and glitching as hell, not worth
Graphicall, need to rewrite almost all parts from scratch. But at least
i now understand almost all details and after some hardcore complex
light scenes tests i am sure it must be finished and included as
experimental feature, at least on CPU.

Bigger showstopper for now is proper support of asymmetric BSDF (smooth
shading normals, bump maps, refraction w/o exit of media), not sure how
to make it better w/o lot of extra code lines, and proper threading and
GPU kernel parameters, to make code at least compile on OpenCL/CUDA.
Also, all old bugs present as only perspective camera, no DOF for direct
light (caustic from glass ball on floor not respect camera lens DOF),
visibility booleans (at least to hide mesh light, many scenes use that),
many other.

BTW, I turn off many features in kernel_types.h, really need only
MULTI_CLOSURES, but it seems broken and i need to turn off many other
unrelated, as HAIR and PROGRESSIVE. It not needed, but problem is i copy
too many data if MULTI_CLOSURE used in Markov chain mutations, it burn
CPU like hell, so i just turn it off. Of course i will optimize that
absurdly big data copied, i doing it as paranoic saving all as i can.

I think i will post a bit better variant in week or two, that little
success give some boost. But for now i busy re-rendering all
collected .blends non stop :)