Blender Git Loki

Cycles: Implement an area preserving parameterization sampling for area lamps

Replace old code for area lamps which was more like incorrect with more correct
one using the following paper as a reference:

Carlos Urena et al.
An Area-Preserving Parametrization for Spherical Rectangles.
https://www.solidangle.com/research/egsr2013_spherical_rectangle.pdf

Implementation is straight from the paper, currently the rectangle contants are
calculated for each of the samples. Ideally we need to pre-calculate them.

The old PDF is still used, which makes difference real subtle. This is to be
corrected before final commit.

Reviewers: brecht, juicyfruit

Subscribers: dingto, ton

Differential Revision: https://developer.blender.org/D823