MaterialX WebGL Path Tracer

Now that the [MaterialX Web Viewer](https://academysoftwarefoundation.github.io/MaterialX/) is available for quick validation of MaterialX content in the browser, an especially valuable next step would be the contribution of a _WebGL Path Tracer_ that generates corresponding path-traced renders.

One potential starting point for this project is the [three-gpu-pathtracer](https://github.com/gkjohnson/three-gpu-pathtracer) by Garrett Johnson, which provides the fundamental building blocks for path-traced rendering using Three.js.

Another good role model is the [OpenPBR Viewer](https://github.com/portsmouth/OpenPBR-viewer) by Jamie Portsmouth, which places a strong emphasis on physical accuracy in rendering, and uses the Standard Shader Ball for consistency with offline renders.

Start by reviewing the MaterialX Web Viewer, three-gpu-pathtracer, and OpenPBR Viewer linked in the issue to understand the proposed direction. Done would be a WebGL path tracer that generates corresponding path-traced renders for MaterialX content in the browser.

Written by the indexing model from the issue text.