Incorrect paths with USD instanceable prims

[jamos05]

When an object is instanceable in USD, we noticed that cryptomatte seems to output its prototype path instead of its instance path, e.g. if there is a instanceable prim called "/geo/sphere", it shows up as "/__Prototype_1".

This means that every instance sharing the same prototype all end up being lumped together in the Cryptomatte output. Also, the prototype number sometimes changes -- while its possible to make it consistent from render to render via the USD_ASSIGN_PROTOTYPES_DETERMINISTICALLY environment variable, the numbers still change whenever any new prototypes are added to the scene.

While it should be possible to workaround these issues by overriding the crypto_* attributes manually, in USD you cannot set attributes directly on instanced scopes, so you have to resort to creating inherit opinions for every single child scope instead, which is a headache and a lot of extra overhead for scenes with thousands of child scopes.

It would be better if Cryptomatte knew how to overwrite the instance path instead, which would avoid these issues.

I'm attaching a simple USDA test scene that exhibits this behavior, with a non-instanced cone and two natively instanced spheres:
native_instances.zip