Summary:
Fix errors raised by issue on GitHub - extending mesh textures + rendering with Gourad and Phong shaders.

https://github.com/facebookresearch/pytorch3d/issues/97

Reviewed By: gkioxari

Differential Revision: D20319610

fbshipit-source-id: d1c692ff0b9397a77a9b829c5c731790de70c09f