EXT_texture_procedurals_mx_1_39 Support Standard Surface

[DennisSmolek]

I get the idea of the system being to match the PBR shader node which makes sense, however MANY of the mtlx materials are made using the standard surface.

The standard shader system needs to map to it but a procedural one doesn't really.

I am working on bringing the extension to threeJS and we already support importing MaterialX mtlx files directly: https://threejs.org/examples/?q=materialx#webgpu_loader_materialx

These are all standard surface files.

I see that the JSON conversion works for "unsupported" graphs however it doesn't generate a material for it nor assign it to assets breaking the gltf.

I suggest a few options:

1. Put the onus on the end user on how they handle standard vs PBR surfaces. By passing this along renderers/loaders will map the output to their system. This is the least impact and lets most standard existing MaterialX assets (both standard surface and GLTF_PBR) work

2. Put conversion into the converter library. If a standard surface is detected, inject The Existing Conversion Node
   The downside with this is it forces all systems to use the conversion library which they may not do.