USD Import: handle normals with uniform interpolation

[kowalskima]

I added logic to import USD normals with uniform interpolation (i.e., per-face normals) and have pushed this change to branch usd-importer-T81257-merge.

I've attached cubeUniformNormals.usda with example uniform normals for testing.

cubeUniformNormals.zip

[kowalskima]

Link to commit:
dff24cf