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.