This patch adds support for light portals: objects that help sampling the
environment light, therefore improving convergence. Using them tor other
lights in a unidirectional pathtracer is virtually useless.
The sampling is done with the area-preserving code already used for area lamps.
MIS is used both for combination of different portals and for combining portal-
and envmap-sampling.
The direction of portals is considered, they aren't used if the sampling point
is behind them.
Reviewers: sergey, dingto, #cycles
Reviewed By: dingto, #cycles
Subscribers: Lapineige, nutel, jtheninja, dsisco11, januz, vitorbalbio, candreacchio, TARDISMaker, lichtwerk, ace_dragon, marcog, mib2berlin, Tunge, lopataasdf, lordodin, sergey, dingto
Differential Revision: https://developer.blender.org/D1133
f478c2cfbd