Update to MaterialX v1.38.5

[pablode]

Description of Change(s)

This PR makes USD (specifically hdStorm) MaterialX 1.38.5-ready.

The changes between 1.38.4 and 1.38.5 are addressed as following:

• the new transmission handling option is set to TRANSMISSION_OPACITY for tests to pass as suggested by @JGamache-autodesk (#929)

• addition of the emission call for said option (#994)

• changes for code generation from material nodes (#981)

• lastly, Jonathan inlined the GlslShaderGenerator::TARGET constant in #926, which doesn't need an #ifguard because the value is the same in both 1.38.3 and 1.38.4

• I have submitted a signed Contributor License Agreement

[sunyab]

Filed as internal issue #USD-7523

[dgovil]

@sunyab were there any updates on this PR or #2324 with regards to USD 23.5?

MaterialX should be bumping up to 1.38.7 soon, and afaik there aren't any API changes since 1.38.5 (.4 was hopefully an anomaly). 1.38.7 has a lot of niceties including MSL support that would be nice to have in a pipeline. Having this PR in would ease a lot of downstream build complexity.

[spiffmon]

Hi @dgovil . We never got to the out-of-band work to get 1.38.5 installed in our vfx2020 platform. We are currently filling out our vfx2022 platform, and 1.38.6 is already installed there. We are still hoping to be able to switch to that platform before summer, but there may still be snags that come up. Would it be better to try to get 1.38.5 now, or try to be ready for the release of 1.38.7 and add that to our vfx2022 platform asap? I don't think we have resources to do both.

[dgovil]

@spiffmon that's good to know. I agree that VFX Platform 2022 is more important.
The reason I pinged this was that we're trying to get MaterialX into Blender against MaterialX 1.38.6, so we were trying to get more info about this particular PR to figure out a plan of action.

[brechtvl]

Maybe this could be merged without the change to build_usd.py? That would add compatibility with newer MaterialX versions without requiring anyone to switch immediately.

Compatibility is all that is needed for Blender and I imagine various other software, where library versions are not dictated by USD.