support for MaterialX-1.38.1

[pmolodo]

Description of Change(s)

Allows building against MaterialX-1.38.1 or 1.38.0

1.38.1 changes the return type of getShaderNodes from an unordered_set to a vector.

I use auto to handle this - I know auto isn't generally encouraged at Pixar, but since MaterialX apparently provides no way to tell the version at compile time, #define isn't an option (and I didn't want to use run-time branching, which would result in a lot of duplicated code!)

Fixes Issue(s)

• Building against MaterialX-1.38.1

[jilliene]

Filed as internal issue #USD-6929