Apply subsurface_bsdf weight in MDL and add tests

[krohmerNV]

This PR is mostly about the added tests materials.
Attached is a comparison between GLSL, OSL, and MDL.

• MDL should be correct as far as I can tell. SSS is implemented using a random walk in the path tracer (which will be added to Add MDL Compiler dependency for [genmdl] tests #2254)
• For the GLSL implementation the radius seems to not have a big impact which might be okay due to the real-time approximation. But we are wondering about the max(mfp, 0.1) here . This means at least 10cm radius if the scene is modelled in meters, right?
• OSL matches GLSL but I haven't checked the source.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: krohmerNV (5a1a9f4, 43bd3a7)
• ✅ login: jstone-lucasfilm / name: Jonathan Stone (5b00a00)

[jstone-lucasfilm]

These look like good changes to me, thanks @krohmerNV.

Our render support for subsurface_bsdf in GLSL and OSL testrender are very early, so I wouldn't yet expect a match with the more mature implementation in MDL, but these new tests should help to align future implementations across all languages.