glTF / MaterialX Converter addition

[kwokcb]

Summary

• Adds the external converter to allows support for bidirectional transfer between glTF and MaterialX for materials. This is embedded as part of MaterialXRender instead of a separate module.
• Adds Python a wrapper and a Python utility script mxgltf.py
• Adds save / load to / from glTF in MaterialX Viewer and MaterialX Graph Editor.

Caveats

• Does not include any tools to perform shader translation nor baking . This can be done as a separate pre-process using the MaterialX Viewer or other means.