Finalize changelog for 1.39.4 (#2559)

Additional changelog update (#2501)

Finalize changelog for 1.39.3 (#2265)

Finalize changelog for release candidate (#2255)

Finalize changelog for 1.39.2 (#2188)

Update changelog for release candidate (#2175)

Backport of the OpenPBR Surface shading model from MaterialX 1.39 to …

…MaterialX 1.38.10

Backport latest translation graphs to 1.38 (#2106)

Adds @mkuo-lucasfilm OpenPBR Surface translation networks to the backport. Also includes tweaks for the USD translation and a small doc update for Standard Surface that was part of the translation graph work.

Backport OpenPBR Surface to MaterialX 1.38.10 (#2082)

Backports only the minimal code necessary to render native OpenPBR surface in MaterialX in a way that has no visual impact on the existing shaders and pbr nodes present in 1.38.

Status:
- GLSL -> Fully functional
- MSL -> Fully functional. Required only backporting the inversesqrt fix on top of the GLSL code.
- OSL -> As functional as the 1.39 branch. Getting it fully functional requires OSL v1.14 features not yet released like Zeltner fuzz and compensating OrenNayar. I was not able to find any mention of a 82 deg color update on the OSL Schlick node though.
- MDL -> Compiles and validates, but no updated visuals until an updated MDL library is available.

Finalize changelog for 1.39.1 (#2001)