Use Render Delegate's material binding pupose

[robpieke]

Description of Change(s)

Changing the SceneIndexAdapterSceneDelegate to query the render delegate's material binding purpose so that the correct material binding is retrieved. This (largely) addresses a regression mentioned below, though there could still be issues with hdPrman and/or Storm which still exercise the old code path.

Fixes Issue(s)

• Largely fixes #3320

• [X] I have verified that all unit tests pass with the proposed changes
• [X] I have submitted a signed Contributor License Agreement

[jesschimein]

Filed as internal issue #USD-10264

[jesschimein]

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