USD: dome light IO

This commit adds logic to convert between USD dome lights and Blender
world materials.

The USD dome light rotation is represented in a mapping node input to the
environment texture.  If the dome light has a color specified in addition to
the texture map, the color will be converted to a vector multiply on the
the environment texture output.

I the imported USD has multiple dome lights, only the first dome light will
be converted to a world material.

Co-authored-by: kiki <charles@skeletalstudios.com>
Co-authored-by: Michael Kowalski <makowalski@nvidia.com>
Pull Request: https://projects.blender.org/blender/blender/pulls/121800

tests/python/bl_usd_export_test.py

@@ -92,6 +92,7 @@ class USDExportTest(AbstractUSDTest):
             filepath=str(export_path),
             export_materials=True,
             evaluation_mode="RENDER",
+            convert_world_material=False,
         )
         self.assertEqual({'FINISHED'}, res, f"Unable to export to {export_path}")