Blender Git Loki

Cycles: Fix for transmissive microfacet sampling

This is an alternate fix for T40964 which resolves bad handling of
caustics reported in T45609.

There were too much transmission rays being discarded by the original
fix, which caused by caustic light being totally disabled. There is
still some room for investigation why exactly original paper didn't
work that well, could be caused by the way how the pdf is calculated.

In any case current results seems rather correct now.