Fix Obj group material propagation: fix #12542

[sebavan]

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Snapshot stored with reference name:
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Test environment:
https://babylonsnapshots.z22.web.core.windows.net/refs/pull/12546/merge/index.html

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Links to test babylon tools with this snapshot:

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https://sandbox.babylonjs.com/?snapshot=refs/pull/12546/merge
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To test the snapshot in the playground with a playground ID add it after the snapshot query string:

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