Add multiscattering terms to diffuse BRDF in image-based lighting

[jjhembd]

Description

This PR adds terms to the diffuse BRDF in image-based lighting, to account for multiple scattering. The math follows a paper by Fdez-Aguera. For a somewhat more gentle introduction, see this practical summary by Bruno Opsenica.

Rendering comparison

Before this PR:

After this PR:

Some key changes to notice:

• Brightening of rough metallic materials, due to multiple scattering.
• Reduction of the diffuse haze on smooth materials, due to more accurate energy conservation in the new formula.
• Less exaggerated Fresnel effect at glancing incidence on dielectric materials, again due to better energy conservation.

Issue number and link

Resolves #12028. (This is the last of a series of fixes addressing that issue).

Testing plan

• Load the "Metal Roughness Spheres" model in the glTF PBR Extensions Sandcastle and compare rendering to the main branch, as well as to the glTF Sample Viewer.
• Load the "Environment Test" model in the glTF PBR Extensions Sandcastle and compare rendering to the main branch and to the results in Bruno Opsenica's blog post.

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
• [ ] I have added or updated unit tests to ensure consistent code coverage See Add screenshot testing for subtle lighting changes #12065
• I have updated the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

[github-actions]

Thank you for the pull request, @jjhembd!

✅ We can confirm we have a CLA on file for you.

[ggetz]

This looks great @jjhembd! I can confirm the effect on both the metal roughness spheres and environment test, and I spot checked the other models against the glTF sample viewer.