* `RUNTIME_LUT_CREATION` is disabled by default, but if set to `true`,
the new `bxdf_lut_frag.glsl` should generate the LUT that is submitted
in this commit. This new LUT is very close to the previously stored one.
* Generate the LUT by Monte-Carlo sampling with \(I = 1/N\sum\frac{f}{p}\),
except that the samples are not random, but mapped from a regular grid,
following previous approach to avoid interpolation artefacts caused by
noise.
* Added comments to make the computed quantities and the LUT usage more
clear.
* Glass with `IOR < 1` is now slightly darker due to single-scattering.
The lost energy will be recovered in a future commit.
* The special cases of `IOR == 0`, `roughness == 0` and `roughness == 1`
are handled well during microfacet sampling and evaluation, no need to
clamp.
* When `IOR == 1`, in theory BRDF is zero and BTDF is one, but we do not
make it a special case to allow smooth transition.
Pull Request: https://projects.blender.org/blender/blender/pulls/111632
9fba9f418d