Blender Git Loki

USD importer: vertex normals render black.

Setting point normals by invoking normal_float_to_short_v3(),
instead of directly assigning components as floats.

USD importer: custom normals shading bug.

Removed call to BKE_mesh_calc_edges() after setting
UVs, as this function is already called after setting
polys. Moreover, the second, redundant call was causing
incorrect viewport shading of previously set custom
normals. Now also setting the ME_AUTOSMOOTH mesh flag
to enable shading with custom normals.

USD importer stage traversal options.

Added new stage traversal options to filter prim import
based on purpose (guide, proxy and render) and whether
to traverse instances with instance proxies. Also modified
the stage traversal logic to avoid a small memory leak due to
creating a reader for the pseudo root which is never used or
deleted.

Fixed compile errors. Registering USD plugins.

Initial USD Import open sourcing.

Cherry-picked commit of Tangent Animation's
USD importer from branch ta-usd-import in
https://github.com/tangent-opensource/blender.git.