ea54cbe1b4
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