Fix UsdValidation reporting errors

[prathikkaranth]

This PR fixes issue #21 created.

So there are 3 validation error types, and 400+ errors of those:

1. InvalidMaterialCollection - 326 errors because the binary geo file (ShdrPlygrnd_Geo.usd) has rel material:binding = None on 359 prims which is explicitly preventing material assignments. Removing that gets rid of these errors.
2. UnresolvableDependency - 110 errors because texture references across 39 mtlx files are textures/ and not relative path to usda so should be ../textures/.
3. InvalidSubsetFamily - 1 error. One thing that is present in a material subLayer is a GeomSubset child override with uniform token familyName = "materialBind" which is not present for any other child. This suggests a duplication as the familyType of the UsdGeomSubsets in this case is of type UsdGeomTokens->partition which means from the docs - every element of the whole geometry appears exactly once in only one of the subsets belonging to the family. So in the ShdrPlygrnd_Geo.usd file we are already specifying the family name for all GeomSubsets so this one is not necessary which is why the validation error appears.

The PR is best reviewed commit by commit as each one presents a solution to each of these errors as the names of the commit suggests.

[linux-foundation-easycla]

• ❌ - login: @prathikkaranth / name: Prathik Karanth. The commit (169cf13, 91e0a64, d3edf82) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please visit our EasyCLA portal and chat with our support bot.