Fix #137973: Add MaterialX version info on USD export

This change adds MaterialX version information to the exported MaterialX
USD materials.

Details:

In USD 25.02, the MaterialXConfigAPI schema was introduced to allow
recording the MaterialX version used to author a material. When loading
MaterialX documents into USD, this schema is automatically applied and
the associated version attribute is created on the material.

Due to how the MaterialX export works (via copying the composed
MaterialX spec from a temporary stage), the MaterialXConfigAPI needs to
be explicitly applied to the final material being exported.

This change applies this MaterialXConfigAPI schema and copies the
version attribute from the temporary MaterialX stage to the final stage.

Authored by Apple: Dan Knowlton

Co-authored-by: Dan Knowlton <d_knowlton@apple.com>
Pull Request: https://projects.blender.org/blender/blender/pulls/137974

tests/python/bl_usd_export_test.py

@@ -9,7 +9,7 @@ import pprint
 import sys
 import tempfile
 import unittest
-from pxr import Gf, Sdf, Usd, UsdGeom, UsdShade, UsdSkel, UsdUtils, UsdVol
+from pxr import Gf, Sdf, Usd, UsdGeom, UsdMtlx, UsdShade, UsdSkel, UsdUtils, UsdVol
 
 import bpy
 
@@ -1132,6 +1132,11 @@ class USDExportTest(AbstractUSDTest):
         material_prim = stage.GetPrimAtPath("/root/_materials/Material")
         self.assertTrue(material_prim, "Could not find Material prim")
 
+        self.assertTrue(material_prim.HasAPI(UsdMtlx.MaterialXConfigAPI))
+        mtlx_config_api = UsdMtlx.MaterialXConfigAPI(material_prim)
+        mtlx_version_attr = mtlx_config_api.GetConfigMtlxVersionAttr()
+        self.assertTrue(mtlx_version_attr, "Could not find mtlx config version attribute")
+
         material = UsdShade.Material(material_prim)
         mtlx_output = material.GetOutput("mtlx:surface")
         self.assertTrue(mtlx_output, "Could not find mtlx output")