Use arnold instancer node in the procedural

[sebastienblor]

With this PR, we read usd PointInstancer primitives as arnold "instancer" nodes (instead of creating lots of ginstance nodes).
We now set the primvars similarly to what usd does.
I'm adding test_0196 where the primvars are set on the parent instancer, that was already working in the render delegate. All the other tests are working correctly.
I noticed the performance is better than in the previous implementation

Issues fixed in this pull request
Fixes #949
Fixes #938
Fixes #956

[sebastienblor]

In the latest commit I'm also fixing the double transformation issues with point instancer reported in #956.
I could have done it in a separate PR but it required this one to be merged first so I'm putting everything under the same hat.

There were 2 problems to be addressed, both happening when an instancer prototype is a nested usd procedural (which we need to do e.g. when the prototype is a xform in the usd scene) :

1. when this prototype has a xform, it was being applied twice. This happened because an instance inherit_xform only applies to the child procedural's matrix, not to its root primitive's matrix. So we need to exclude the prototype matrix in that use case.

2. these child usd procedurals have a root primitive (object_path set in the procedural) so that it only loads the expected prototype. But when we compute primitive's matrices in this procedural, we consider the ones from the root prim's ancestors, which shouldn't happen