This is an initial implementation of a USD importer.
This work is comprised of Tangent Animation's open source USD importer,
combined with features @makowalski had implemented.
The design is very similar to the approach taken in the Alembic
importer. The core functionality resides in a collection of "reader"
classes, each of which is responsible for converting an instance of a
USD prim to the corresponding Blender Object representation.
The flow of control for the conversion can be followed in the
`import_startjob()` and `import_endjob()` functions in `usd_capi.cc`.
The `USDStageReader` class is responsible for traversing the USD stage
and instantiating the appropriate readers.
Reviewed By: sybren, HooglyBoogly
Differential Revision: https://developer.blender.org/D10700
