"Enable Scene Materials" in usdview doesn't work with RenderMan

[robp-sidefx]

I noticed that with USDIMAGINGGL_ENGINE_ENABLE_SCENE_INDEX=1, the "Enable Scene Materials" menu item doesn't take immediate effect when the RenderMan delegate is active. If I switch to Embree and back, I can see the materials have then appeared/disappeared.

This still works as-expected (i.e., live update) with USDIMAGINGGL_ENGINE_ENABLE_SCENE_INDEX=0. It also works as-expected in Storm, perhaps due to the Storm-specific Scene Index that links materials and meshes for dirtying/syncing (though this is purely speculation at this point).

We're tracking down some similar misbehaviour in our app, which is what steered me towards "let's see if it's actually working in usdview" as a debugging step.

[jesschimein]

Filed as internal issue #USD-10590

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