Update testrender implementation of microfacet models to include the
improved visible normal sampling technique of Eugene d'Eon and Eric
Heitz (with improvements to numerical stability and continuity over the
code provided in the paper)

Add a stratified sampler to replace the basic monte carlo sampling
routine

Further simplify the math of the Oren-Nayar implementation

Switch to fast math routines to improve the consistency of results
across platforms