Fix Standard surface -> glTF PBR translation: thin_walled and transmission_depth

[bowald]

Thickness in glTF PBR is used to describe how thick a mesh is. If it's thin walled, thickness is 0, otherwise it should be a value corresponding to a mesh, in a ray tracer, you will have that info when rendering, for a rasterizer you either bake it or use an arbitrary value (such as longest side of the bounding box).

Since Thickness is a mesh related value, we can't translate to the expected value, unless the material is considered thin_walled. Then it should be 0. I have updated the translation graph from Standard surface to set thickness to 0 if thin walled, otherwise 1 (which is better then nothing).

Previously transmission_depth was used for thickness, which do not correspond to glTF PBRs thickness. I have changed it to be used for attenuation_distance which correspond to transmission_depth, (with the caveat that in glTF PBR it should >0, as base_color is used to color the surface even for thin_walled transmissive materials).

From Standard Surface - 3.8 Specular Transmission

From KHR_materials_volume

[jstone-lucasfilm]

This looks promising, @bowald, and I'll just highlight the validation error that shows up in GitHub CI:

Shader translation of D:\a\MaterialX\MaterialX\build\bin\resources\Materials\Examples\StandardSurface\standard_surface_brass_tiled failed
Validation errors: Invalid port connection: <output name="thickness_out" type="float" nodename="thickness_from_thin_wall">
Invalid port connection: <output name="thickness_out" type="float" nodename="thickness_from_thin_wall">

[bowald]

Thanks for pointing it out @jstone-lucasfilm.
I changed the node name last thing I did. Great that the CI system catches these thing 👍