- In fresnel_dielectric, the differentials calculation sometimes divided by zero.
- When the normal map was (0.5, 0.5, 0.5), the code would try to normalize a zero vector. Now, it just uses the regular normal as a fallback.
- The approximate error function used in Beckmann sampling sometimes overflowed to inf while calculating r^16. The final value is 1 - 1/r^16, however,
so now it just returns 1 if the computation would overflow otherwise.
Our own implementation is in fact the same performance as in fast_math from
OpenShadingLanguage, but implementation from fast_math is using explicit madd
function, which increases chance of compiler deciding to use intrinsics.
