Improvements to GGX importance sampling

[jstone-lucasfilm]

• Implement the paper "Sampling Visible GGX Normals with Spherical Caps" by Jonathan Dupuy and Anis Benyoub, which improves the performance and spatial continuity of VNDF sampling.
• Switch to VNDF sampling for FIS environment lights, improving the convergence of this lighting path for lower sample counts.
• Additional optimizations for FIS environment lights, leveraging the improved term cancellation in VNDF sampling.

Test results:

• GLSL render performance is improved for all tested materials, e.g. an increase from 205 fps to 214 fps for standard_surface_default.mtlx with 16 environment samples on an NVIDIA RTX A6000.
• Convergence of FIS environment lights is improved for all tested materials, with the maximum visual error between 16 and 16384 environment samples reduced from 0.1098 to 0.0745 for a rough gold material.

[jstone-lucasfilm]

Adding some render comparisons and statistics:

Original Filtered Importance Sampling (16 samples / 16384 samples / difference image)
Maximum visual difference: 0.1098
Average visual difference: 0.0150

New Filtered Importance Sampling (16 samples / 16384 samples / difference image)
Maximum visual difference: 0.0745
Average visual difference: 0.0127

[jstone-lucasfilm]

And here are the latest render comparisons between GLSL and OSL:
MaterialXRenderTests_06_28_2023.pdf

[pixnblox]

Looks good, thanks for getting this in! 👍