USD Import: normals import issues

[kowalskima]

I noticed the following issues when importing USD normals. I've implemented fixes to each of these items in branch usd-importer-T81257-merge.

• Redundant call to BKE_mesh_calc_edges() after setting face-varying normals was causing incorrect shading.
• Vertex normals were rendering black. Fixed by invoking normal_float_to_short_v3() to assign the vector, instead of directly assigning components as floats.
• Empty vertex-varying normals were causing black renders. Now invoking BKE_mesh_calc_normals() when point normals are empty. Since BKE_mesh_calc_normals() requires edges to be defined, moving point normal calculation to follow reading polys.
• Now handling primvars:normals if specified. Per Pixar UsdGeomPointBased documentation: If 'normals' and 'primvars:normals' are both specified, the latter has precedence. http://graphics.pixar.com/usd/docs/api/class_usd_geom_point_based.html#ac5427209fe38570a75036a3cb7ebb553
• Uninitialized USDMeshReader::m_isLeftHanded flag was causing the vertex winding orientation to randomly flip, causing incorrect shading and other issues.
• Orientation was ignored when assigning normals, causing incorrect shading (see attached image showing incorrect rendering of the attached example sphereLeftHandedOrientation.usda).

sphereLeftHandedOrientation.zip