Usd2gltf

usage: usd2gltf [-h] [-i INPUT] [-o OUTPUT] [--interpolation INTERPOLATION] [-d] [-f]
optional arguments:
- -h, --help show this help message and exit
- -i INPUT, --input INPUT
- Input USD (.usd, .usdc, .usda, .usdz)
- -o OUTPUT, --output OUTPUT
- Output glTF (.gltf, .glb)
- --interpolation INTERPOLATION
- Interpolation of animation (LINEAR, STEP, CUBIC)
- -d, --debug Run in debug mode
- -f, --flatten Flatten all animations into one animation

Fix - The cli works for the most part with a usdz input path. But if this module is used as part of a workflow and the stage is passed to the converter instead of the path we need a way to handle any usdz files that exist as layers.
This is a quick and dirty solution. We check all loaded layers and return the first layer that contains a usdz path.
Ideally, we want to unzip all usdz paths and recreate the stage with the new references.

Fixes:
* Exporter would fail if primvars had a different total to the FaceVertexCount. Using ComputeFlattened for now.
* Material index for mesh primitive wasn't being assigned for certain subsets.

Quick fix - The log would output all subset indices if debugging was enabled which wasn't particularly useful with high poly count models.

Removing shebang from cli to see if that fixes an issue with running the command on Windows.

Added support for skeletons and skinned animation.