Added vertex colour utility node and translation for USD preview surface

[jonjondev]

This PR adds vertex colour functionality to the USD preview surface shader by adding a cpvColor utility node and implementing its connection as part of the USD translation of the displayColor primvar reader. While initial implementations utilised the MPxShaderOverride to add displayColor as a feature of the USD preview surface node itself, we eventually landed on adding it as a separate utility node, so that it could more closely mimick USD shade's functionality, and provide greater flexibility.

The images below demonstrate it in action (left is lambert shading, middle is preview surface shading, right is proxy shape):

[JGamache-autodesk]

Nice and covers the most frequent use case. Thanks!

[JGamache-autodesk]

The unit tests fail because test/lib/usd/translators/testUsdImportPreviewSurface.py is executed without UI in mayabatch.
Please move the unit test to test\lib\mayaUsd\render\vp2RenderDelegate\testVP2RenderDelegateUSDPreviewSurface.py which is a Maya-interactive test.

[jonjondev]

The unit tests fail because test/lib/usd/translators/testUsdImportPreviewSurface.py is executed without UI in mayabatch. Please move the unit test to test\lib\mayaUsd\render\vp2RenderDelegate\testVP2RenderDelegateUSDPreviewSurface.py which is a Maya-interactive test.

Ah! Thanks for that. I'd been trying to figure out why it couldn't find any panels.