Use “specular occlusion” term to consistently extinguish fresnel on Ambient and Environment Map lights

[coreh]

Objective

Even at reflectance == 0.0, our ambient and environment map light implementations still produce fresnel/specular highlights.

Such a low reflectance value lies outside of the physically possible range and is already used by our directional, point and spot light implementations (via the fresnel() function) to enable artistic control, effectively disabling the fresnel "look" for non-physically realistic materials. Since ambient and environment lights use a different formulation, they were not honoring this same principle.

This PR aims to bring consistency to all light types, offering the same fresnel extinguishing control to ambient and environment lights.

Thanks to @nathanf for pointing out the Filament docs section about this.

Solution

• We use the same formulation already used by the fresnel() function in bevy_pbr::lighting to modulate the F90, to modulate the specular component of Ambient and Environment Map lights.

Comparison

⚠️ Modified version of the PBR example for demo purposes, that shows reflectance (NOT part of this PR) ⚠️

Also, keep in mind this is a very subtle difference (look for the fresnel highlights on the lower left spheres, you might need to zoom in.

Before

After

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Changelog

• Ambient and Environment Map lights will now honor values of reflectance that are below the physically possible range (⪅ 0.35) by extinguishing their fresnel highlights. (Just like point, directional and spot lights already did.) This allows for more consistent artistic control and for non-physically realistic looks with all light types.

Migration Guide

• If Fresnel highlights from Ambient and Environment Map lights are no longer visible in your materials, make sure you're using a higher, physically plausible value of reflectance (⪆ 0.35).

[Diddykonga]

LGTM, although I am no graphics wizard, just that it literally looks good to me.