Enable alpha test when using vertex alpha in some cases

[bghgary]

This change addresses some edge cases tested by a new test model in the glTF sample assets repo: https://github.com/KhronosGroup/glTF-Sample-Assets/pull/146.

The issue is that alpha test is not taking into account whether a mesh has vertex alpha or not.

Before:

After:

TODO

• [ ] Add test once new test model is merged

[bjsplat]

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[bjsplat]

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[github-actions[bot]]

This pull request has been marked as stale because it has been inactive for more than 14 days. Please update to "unstale".

[bghgary]

Closing in favor of #15573