* Using standard NDF and Smith shadowing-masking terms. The previous
`xxxx_opti()` functions were faster to evaluate, but confusing and
error-prone.
* After correcting the BRDF pdf, the prefiltered environment LOD bias
needs to be adjusted to avoid overblurred reflections.
* Corrected the half-vector computation in BTDF evaluation, added check
for invalid configuration due to total internal reflection or `eta == 1`.
Weizhen Huang