MAYA-102319 - Material import options rework

[JGamache-autodesk]

The major item of this work is to remove the material import dropdown in favor of a single checkbox for importing materials or not. The strategy is to provide a list of shadingModes to try in the import options and let the code execute each one in sequence until a material is found.

Major work:
The shadingMode import option is now a list of shading modes to try.

• Added code to extract array of tokens from options string.
• Added code to generate options string containing arrays
• The mayaUSDListShadingModes command will list the modes in a predefined order that tries more complex shading modes first.
• The pxrRis shadingMode import has been made more strict to let other shading modes succeed.
• Shading modes that declare a renderContext will no longer explore the universal context. This allow universal context shading modes importers to successfully run.

Added materialConversion support for import modes. This mirrors the export mode exactly.

• Added registry functions to register an importer with materialConversion.
• Shares and uses the renderContext concept used for export.
• Renamed shadingConversion to preferredMaterial to prevent confusion. This represents an optional "convertTo" that can be added on top of the "convertFrom" that materialConversion represents.

Other work:
The displayColor shading mode is removed in favor of a better handling of the exportDisplayColor geometry option. This fixes #520.

• The exportDisplayColor now default to false
• The exportOptions MEL script has been reworked to still present a Display Color material export that toggles the exportDisplayColor import option.

Minor work:

• SetPlugValue can now handle 4x4 matrices
• UsdPreviewSurface added to the list of preferred materials on import. Support for displayColor import/export added.
• PhongE removed from list of preferred materials on import.
• Symmetric shader writer now skips hidden attributes.
• vp2RenderDelegate will convert linear displayColor to Maya to provide better color match between the original lambert and the displayColor.

[mattyjams]

Looks good to me! I had a few minor notes, but nothing too critical.

I have to admit that just reading through the code, it's a little confusing to understand how the conversion aspect of it works, but I'm hoping that becomes clearer once we start playing with it.