Cycles: Remove MultiGGX code, replace with albedo scaling

While the multiscattering GGX code is cool and solves the darkening problem at higher roughnesses, it's also currently buggy, hard to maintain and often impractical to use due to the higher noise and render time.

In practice, though, having the exact correct directional distribution is not that important as long as the overall albedo is correct and we a) don't get the darkening effect and b) do get the saturation effect at higher roughnesses.

This can simply be achieved by adding a second lobe (https://blog.selfshadow.com/publications/s2017-shading-course/imageworks/s2017_pbs_imageworks_slides_v2.pdf) or scaling the single-scattering GGX lobe (https://blog.selfshadow.com/publications/turquin/ms_comp_final.pdf). Both approaches require the same precomputation and produce outputs of comparable quality, so I went for the simple albedo scaling since it's easier to implement and more efficient.

Overall, the results are pretty good: All scenarios that I tested (Glossy BSDF, Glass BSDF, Principled BSDF with metallic or transmissive = 1) pass the white furnace test (a material with pure-white color in front of a pure-white background should be indistinguishable from the background if it preserves energy), and the overall albedo for non-white materials matches that produced by the real multi-scattering code (with the expected saturation increase as the roughness increases).

In order to produce the precomputed tables, the PR also includes a utility that computes them. This is not built by default, since there's no reason for a user to run it (it only makes sense for documentation/reproducibility purposes and when making changes to the microfacet models).

Pull Request: https://projects.blender.org/blender/blender/pulls/107958

CMakeLists.txt

@@ -496,6 +496,7 @@ option(WITH_CYCLES_DEBUG             "Build Cycles with options useful for debug
 
 option(WITH_CYCLES_STANDALONE        "Build Cycles standalone application" OFF)
 option(WITH_CYCLES_STANDALONE_GUI    "Build Cycles standalone with GUI" OFF)
+option(WITH_CYCLES_PRECOMPUTE        "Build Cycles data precomputation tool" OFF)
 
 option(WITH_CYCLES_HYDRA_RENDER_DELEGATE "Build Cycles Hydra render delegate" OFF)
 
@@ -507,6 +508,7 @@ mark_as_advanced(WITH_CYCLES_KERNEL_ASAN)
 mark_as_advanced(WITH_CYCLES_LOGGING)
 mark_as_advanced(WITH_CYCLES_DEBUG_NAN)
 mark_as_advanced(WITH_CYCLES_NATIVE_ONLY)
+mark_as_advanced(WITH_CYCLES_PRECOMPUTE)
 
 # NVIDIA CUDA & OptiX
 if(NOT APPLE)