Add function to save mesh image

[kosuke55]

Thanks for the great work.

The image of the mesh can be saved by this PR. A black image may be saved when visible=False due to the issue of trimesh. So I set visible=True and the viewer will start up only for a moment. https://github.com/NVIDIAGameWorks/kaolin/compare/master...kosuke55:save_mesh_image?expand=1#diff-5144965960dbb6e79069588bcace5337R106

import kaolin as kal

mesh = kal.rep.TriangleMesh.from_obj('model.obj')
mesh.save_image('sphere')

sphere.png