Viewer IBL shadow fixes

[MiiBond]

No description provided.

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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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[sebavan]

LGTM but let s have @ryantrem checking it

[ryantrem]

Yep I've reviewed and tested locally. I pushed a couple changes to this PR, but I'd like @alexandremottet to look just to be sure, and then we can merge.