Incorrect material tag detection for OpenPBR surface

[JGamache-autodesk]

Description of Issue

The material tag detection code added in #3073 had some forward looking code for OpenPBR that requires an update, even though we have not yet integrated MaterialX 1.39.

When I first wrote the code, the geometry_opacity parameter was a GfVec3. In the final specification, it is now a float. That is a one line change to do in _GetOpenPBRSurfaceMaterialTag() when integrating MaterialX 1.39, replacing GfVec3f(1.0f, 1.0f, 1.0f) with 1.0f.

Steps to Reproduce

1. None yet. Requires MaterialX 1.39 to get an OpenPBR surface node

System Information (OS, Hardware)

Package Versions

Build Flags

[jesschimein]

Filed as internal issue #USD-9863