Scale transmission roughness by IOR

[issam3105]

Objective

• Scale transmission roughness by IOR.
• In the Khronos TransmissionRoughnessTest sample, Khronos' comment explains the core case:
  With IOR = 1.0, light vectors do not change regardless of how strong the microfacet angles become. When this happens, transmission will be sharp and non-blurred, regardless of the material's roughness, because the microfacets cannot alter the direction of transmitted light.

Solution

• Add an ior_corrected_roughness helper matching the Khronos glTF Sample Renderer behavior

Testing

• Tested visually with the Khronos TransmissionRoughnessTest glTF sample: the 1.0 - Air row should remain sharp across roughness values.

Showcase

• Before:

• After:

• Blender Cycles

[goodartistscopy]

Looks sensible to me and in line with refraction occurring on microfacets.

[dylansechet]

Main	This PR	Cycles reference

Looks good to me! The Khronos formula doesn't correct for IOR>1.5, but that was already an issue before this PR.

[goodartistscopy]

Looks good to me! The Khronos formula doesn't correct for IOR>1.5, but that was already an issue before this PR.

Indeed, this could be tweaked by changing the upper bound on the clamp() I believe.