Blender Git Loki

Fix T47411: Cycles portals luminance artifacts

When using multiple portals, scene areas behind one of the portals were rendered darker than they should.
The reason for that is a pretty stupid mistake: Since portals are only used at positions that aren't behind them,
only portals that are used should be accounted for in the PDF calculation. That was actually the case, but the final
divide incorrectly divided by the total amount of portals, not the amount of visible ones.

Another issue with areas behind portals was the PDF evaluation function.
The new evaluation code is shorter, simpler and fixes this issue.

Also, the threshold for the distance check was increased to avoid artifacts where portals touch a surface.