USD Import: crash on reading no polys #27
Description
To reproduce, select the Global Read Flag option to Vertex only and load a USD containing a mesh.
I've committed this fix to the usd-importer-T81257-merge branch: c77832d
Crash occurs because downstream code expects polys to exist. Modified the mesh import logic to always read verts and polys for new meshes, regardless of the value of the read flag.