Implement PBR Maps Node

[blessedcoolant]

A node to generate normal, roughness and displacement maps from a single image. Based on: https://github.com/joeyballentine/Material-Map-Generator

[hipsterusername]

@psychedelicious @RyanJDick @brandonrising - I imagine we want to handle these models differently, thinking about new user UX (and hosting these)?

[blessedcoolant]

I think it's better we host them ourselves. The Github downloads can take ridiculously long sometimes for no apparent reason. The UI use case would be currently right click on an image and say "Generate PBR Maps" from it. Ideally we want to do a proper 3D viewer for previewing the model but that's an entirely different feature.