Cycles: better distance sampling for chromatic volume extinction.

Previously we picked one of the RGB channels with equal probability, but this
works poorly in a dense volume after many bounces. Now we take into account
the throughput and single scattering albedo.

This makes it a little more practical to do brute force SSS with volumes, but
is still very inefficient because we do direct light sampling at every volume
bounce even when inside an opaque mesh. In theory there could be a light inside
the mesh so we can't automatically disable direct lighting.

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