Hydra 2: different behavior with normals for UsdSkel vs Hydra 1

[cameronwhite]

Description of Issue

In Hydra 2, we observed that normals on skinned meshes are passed through unmodified, whereas in Hydra 1 the normals were blocked and forced to be recomputed by the render delegate after the skinning computation. Depending on the renderer's settings, the Hydra 1 behavior might produce faceted or smooth normals (as mentioned in #3406), but this at least avoids obvious shading issues from using the rest geometry's normals as-is

Note that this will have a proper fix once PRs such as #3664 land in the future to add support for skinning normals, but I'll still post a PR with my patch to match the Hydra 1 behavior in case it's useful in the meantime.

Steps to Reproduce

• Download the attached scene test_lbs_normals.usda.zip which adds normals to test_lbs.usda from the USD repo
• Run env USDIMAGINGGL_ENGINE_ENABLE_SCENE_INDEX=0 usdview test_lbs_normals.usda --camera /cameras/camera1 --cf 86, and with USDIMAGINGGL_ENGINE_ENABLE_SCENE_INDEX=1 to compare the results with Hydra 2 (attached below are the results)

Hydra 1:

Hydra 2:

[tpalmergit15]

Filed as internal issue #USD-11174

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