glTF Exporter: Add multimaterial test

[alexchuber]

Follow-up to #17241.
Tests export of multimaterials / submaterials / submeshes.

My ulterior motive is to add a glTF exporter test that 1. doesn't use `File` and 2. doesn't use an existing image as a texture. That way, it can double as a smoke test for BN.

[bjsplat]

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To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

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[bjsplat]

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[bjsplat]

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[bjsplat]

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[alexchuber]

Azure outage is probably causing CI failure. Will re-run when it's back up.

[bjsplat]

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[bjsplat]

Visualization tests for WebGPU

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