FIX issues with FBX files exported by 3ds Max 2021

[susares]

3ds Max 2021 introduced an undocumented change when exporting physical materials to FBX.
Which leaves the MaterialDescription based import unable to decide if it is a physical or Arnold material.

Basicly 3ds Max 2021 writes out the Arnod Standard Surface material ClassId instead of the one for Physical Materials.
It also adds a new ORIGINAL_MTL property which hints to the physical material.
Sadly in that case instead of writing Arnold related properties. Physical Material properties are written making

FBXMaterialDescriptionPreprocessor assuming it is an Arnold Standard Surface Material unable to read out any specialized properties.

The following pull request works around this change.

Autodesk is currently investigating this issue but there is no info on if or when a fix will be released.

[susares]

I forgot to mention, that this will affect all Unity3D Versions currently released that support material description based FBX imports.
When then try to import 3ds Max 2021 exported FBX files.

[aras-p]

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