Improve sampling of area lights
Improve sampling of area lights

This patch uses the sampling method described in "A Low Distortion Map Between Triangle and Square" by Eric Heitz.
The benefit is avoiding sqrt in the calculation, which could be cheaper on some architectures, and the result is
more even sampling across the triangle surface.

Based on ideas from
https://pharr.org/matt/blog/2019/02/27/triangle-sampling-1.html
https://pharr.org/matt/blog/2019/03/13/triangle-sampling-1.5.html

Reviewed By: Brecht Van Lommel

Differential Revision: https://developer.blender.org/D6566

tangent-opensource/coreBlackbird,tangent-opensource/coreBlackbird,tangent-opensource/coreBlackbird