[3.x] Add maximum roughness cutoff to SSR to improve performance

[Calinou]

3.x version of #56804.

In a test scene with mixed rough and non-rough materials, this saves 0.2 ms of GPU time with very little visual artifacting (GeForce RTX 4090, 3840×2160). This also incorporates the roughness fadeout from #69828, which can help hide visual artifacts on rough surfaces and generally make SSR look more stable.

The roughness cutoff can be adjusted in the Environment properties. Setting it to 1.0 will result in the previous behavior being used.
Edit (June 2023): This is no longer present for now. We need to discuss whether we should add this setting, as it's not in 4.x and probably won't be added there.

Testing project (3.x): test_ssr_performance-3.x.zip

Preview

No roughness cutoff (previous behavior)

Roughness cutoff = 0.7 (new default)

[clayjohn]

I'm not sure if adding this to 3.x makes sense. This will add a sharp cutoff point where SSR will just disappear. In 4.0 the equivalent code was added after we added a smooth falloff to the SSR rough reflections so that SSR could gradually fade out on materials with a varying amount of roughness. A similar fade-out could be added here and then I guess it would be fine

[Calinou]

A similar fade-out could be added here and then I guess it would be fine

Yes, I agree backporting both the fade-out and roughness cutoff makes sense. SSR looking too sharp and too present in 3.x is a common complaint after all, so better make it more subtle when it makes sense (and also faster at the same time).

[Calinou]

Rebased and tested again, it works as expected. I've implemented @clayjohn's roughness cutoff from 4.x. However, this poses a challenge for the max_roughness setting, as the fadeout values don't depend on the roughness cutoff. I've tried min()/max()ing those with the max_roughness values, but it tends to look pretty bad in practice if you change the roughness cutoff from the default.

Screenshots in OP were updated to show the new visual and performance results.