Enhancement: 2 layer parallax for PBR shader

[lenship2]

What would this enhancement be for?

Rendering

Describe your enhancement suggestion in more detail

This would effectively serve the same function as $localrefract, but with proper lighting using the PBR shader (and not restricted to models): one material is visually overlayed on top of another material, and that bottom material transforms in accordance with viewer parallaxing.

Here's an example of the shader from Half Life Alyx, where it's used to make a window with parallaxed curtains:

I think an effective way to implement this would be through the second texture slots usually reserved for blend textures, where the second textures are instead used for the material to overlay on top of the parallaxing material. A new vmt parameter like $2layerparallax could be added to enable this behavior.

This could be used for a PBR lightstrip texture for example, where the first texture set is the light emitting bit itself, and the second texture set is the glass cover on top of the lightstrip.

[Smaedd]

I personally would really like to avoid PBR becoming an ubershader so I'm happy for us to implement this but I think it should be in a separate shader and not just regular PBR

[lenship2]

having it as a separate shader would be fine