ensure that indirect sheen is shadowed by specular ambient occlusion. #26940
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Fixed #26939
Description
This fixes sheen so that that when it is lit by indirect light (IBL) it will be shadowed by ambient occlusion.
Before:
![Screenshot 2023-10-10 at 5 01 25 PM](https://cdn.statically.io/img/private-user-images.githubusercontent.com/588541/274103463-667edc9f-8c55-4287-bf52-bd8f8fdef0cc.png?jwt=eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpc3MiOiJnaXRodWIuY29tIiwiYXVkIjoicmF3LmdpdGh1YnVzZXJjb250ZW50LmNvbSIsImtleSI6ImtleTUiLCJleHAiOjE3MjI3MjY5NDAsIm5iZiI6MTcyMjcyNjY0MCwicGF0aCI6Ii81ODg1NDEvMjc0MTAzNDYzLTY2N2VkYzlmLThjNTUtNDI4Ny1iZjUyLWJkOGY4ZmRlZjBjYy5wbmc_WC1BbXotQWxnb3JpdGhtPUFXUzQtSE1BQy1TSEEyNTYmWC1BbXotQ3JlZGVudGlhbD1BS0lBVkNPRFlMU0E1M1BRSzRaQSUyRjIwMjQwODAzJTJGdXMtZWFzdC0xJTJGczMlMkZhd3M0X3JlcXVlc3QmWC1BbXotRGF0ZT0yMDI0MDgwM1QyMzEwNDBaJlgtQW16LUV4cGlyZXM9MzAwJlgtQW16LVNpZ25hdHVyZT1jYmQyYzlkOTg1MGYzNTM2NzM2OTMxMGE5MWQyNmQwMDM4ZjMzYzQyNWE2ZmJkYTNmOGYwYjdiMTFhYmE2OGQ3JlgtQW16LVNpZ25lZEhlYWRlcnM9aG9zdCZhY3Rvcl9pZD0wJmtleV9pZD0wJnJlcG9faWQ9MCJ9.IMZtizk51jfJI0TSOJv2VjnKENXcLiguzPXnZTBgTfs)
After:
![Screenshot 2023-10-10 at 8 26 38 PM](https://cdn.statically.io/img/private-user-images.githubusercontent.com/588541/274103444-03e88b2e-9315-42cd-ac38-fe69d02399be.png?jwt=eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpc3MiOiJnaXRodWIuY29tIiwiYXVkIjoicmF3LmdpdGh1YnVzZXJjb250ZW50LmNvbSIsImtleSI6ImtleTUiLCJleHAiOjE3MjI3MjY5NDAsIm5iZiI6MTcyMjcyNjY0MCwicGF0aCI6Ii81ODg1NDEvMjc0MTAzNDQ0LTAzZTg4YjJlLTkzMTUtNDJjZC1hYzM4LWZlNjlkMDIzOTliZS5wbmc_WC1BbXotQWxnb3JpdGhtPUFXUzQtSE1BQy1TSEEyNTYmWC1BbXotQ3JlZGVudGlhbD1BS0lBVkNPRFlMU0E1M1BRSzRaQSUyRjIwMjQwODAzJTJGdXMtZWFzdC0xJTJGczMlMkZhd3M0X3JlcXVlc3QmWC1BbXotRGF0ZT0yMDI0MDgwM1QyMzEwNDBaJlgtQW16LUV4cGlyZXM9MzAwJlgtQW16LVNpZ25hdHVyZT0yZWZlNzhlMTRhMGFjOWYxMWUwNjE4ZTU2ODU5ODAyODE4YmEyYWE4M2U3ZDdkYWY0YjY2MmUwOTdiOWRjMGRiJlgtQW16LVNpZ25lZEhlYWRlcnM9aG9zdCZhY3Rvcl9pZD0wJmtleV9pZD0wJnJlcG9faWQ9MCJ9.RTG44XfkLwM2b4c_N-2NI1wabtA7MsuCAu7O_TYQV8c)
Caveat: My reuse of the specularOcclusion function is an approximation. I am not sure if it is appropriate for indirect specular, rather I think it was designed for direct specular. It is close anyhow and better than not occluding indirect sheen.
Fun fact: I think that clear coat has the same bug. I will make a PR for occluding indirect clearcoat separately.
This contribution is funded by Threekit