Add reference images

It would be great to have a set of reference images for each layer and each common combination of layers for which renderers who add an implementation of this shader could test their implementation against for look consistency.

Certainly, some implementation may choose to use another specular BRDF or expose some additional closure parameters that the reference implementation lacks.
But when those are at default values the produced look should match the reference implementation as closely as possible.

Only then will this be useful for stuff like sharing shading networks that have this as the final node between renderers/DCC packages or cross facility asset transfer.