Apple : Compilation support for MaterialX 1.38.X and 1.39.X

[ld-kerley]

Description of Change(s)

MaterialX 1.39.X introduced API changes, this PR allows OpenUSD to be built against either MaterialX 1.38.X or 1.39.X.

Most of the changed arise from an API change around MaterialX::TypeDesc being passed by value instead of pointer. The other change is the complete removal of the channels attribute.

NOTE : The MaterialX versioning PR (#3157) should be merged before this.

Fixes Issue(s)

Fixes compile issues when using MaterialX 1.39.X

• [ ] I have verified that all unit tests pass with the proposed changes

• [X] I have submitted a signed Contributor License Agreement

[jesschimein]

Filed as internal issue #USD-9837

:heavy_exclamation_mark: Please make sure that a signed CLA has been submitted!

[jesschimein]

/AzurePipelines run

[azure-pipelines[bot]]

Azure Pipelines successfully started running 1 pipeline(s).

[jesschimein]

/AzurePipelines run

[azure-pipelines[bot]]

Azure Pipelines successfully started running 1 pipeline(s).

[zwillikon]

May you please rebase materialXFilter.cpp ?

[nyabinary]

bump :p

[jesschimein]

/AzurePipelines run

[azure-pipelines[bot]]

Pull request contains merge conflicts.