Summary:
When there is no "usemtl" statement in the .obj file use material from .mtl if there is one.
https://github.com/facebookresearch/pytorch3d/issues/1068

Reviewed By: bottler

Differential Revision: D34141152

fbshipit-source-id: 7a5b5cc3f0bb287dc617f68de2cd085db8f7ad94