Pathtracer Demo

[pshriwise]

There was a request for a path tracer demo on Discord. Seems like it would be a useful one to include. It would be nice to have it so others could easily build on it to create more realistic visualizations fairly easily.

[natevm]

For future reference, this path tracer here could be adapted into GPRT for a sample:

https://www.gsn-lib.org/apps/raytracing/index.php?name=example_transmission

More here: https://www.gsn-lib.org/docs/nodes/raytracing.php