Enable blending for transparent objects

[sirpalee]

Description (this won't be part of the changelog)

Enabling blending for the proxy shape, so semi-transparent objects show up properly in the viewport.

Changelog

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Enabling blending for transparent objects while displaying the Proxy Shape.

Added

Changed

Deprecated

Removed

Fixed

Checklist (Please do not remove this line)

• [ ] Make sure the Title and Description of the PR make sense and provide sufficient context for your work
• [ ] Do any added files have the correct AL Apache Licence Header?
• [ ] Are there Doxygen comments in the headers?
• [ ] Are any new features, behaviour changes documented in the .md format documentation?
• [ ] Have you added, updated tests to cover new features and behaviour changes?
• [ ] Have you filled out at least one changelog entry?

[sirpalee]

I'm going to re-engineer the solution as new blending states have landed in the latest commits of USD.

[sirpalee]

For now I'm closing the PR, since there is a significant change in USD affecting AL_USDMaya. (usdImaging/usdImagingGL/hdEngine.h is a private header now) Once AL_USDMaya is compatible with the latest USD dev branch, we'll reevaluate if setting up the states is necessary.