Make Fresnel darken SSR instead of blending with specular

[mandryskowski]

This PR fixes (partially) #79549 due to Fresnel incorrectly blending SSR with specular, but only if fade in and fade out are set to 0.0.
Without fade in/fade out, the reflections are fully opaque and look correct.

This PR with fade in and fade out set to 0.0:

To avoid specular light blending/passing through SSR reflections, we cannot allow the alpha value of final_color in the SSR shader to drop below 1.0.
Now fade in/fade out introduces blending with specular light. Because the sun in the HDR image is so strong (the brightest sun pixel is RGB(41184.0, 35776.0, 28624.0) compared to around RGB(0.4, 0.4, 0.4) in the SSR reflection), even mixing just a bit of specular light with SSR causes a significant change in color and produces bad results.

This PR with fade in 0.001 and fade out 0.0:

Not sure how we can fix/if we should fix the fade mixing. Maybe there is a better way to mix HDR values than glsl mix() in specular_merge.glsl.

[Calinou]

Now fade in/fade out introduces blending with specular light. Because the sun in the HDR image is so strong (the brightest sun pixel is RGB(41184.0, 35776.0, 28624.0) compared to around RGB(0.4, 0.4, 0.4) in the SSR reflection), even mixing just a bit of specular light with SSR causes a significant change in color and produces bad results.

There is a Clamp HDR Exposure import option that should be enabled on the panorama texture in the Import dock. This is recommended when using real life-sourced panorama images.

[clayjohn]

After the explanation, this makes sense to me. I was wrong in my earlier concern. This should be good to merge

To be even more clear, there remain issues with the source file because the HDRI is way too bright. So it's not expected that this PR will make that file look perfect.

No need to cherrypick for 4.1

[clayjohn]

The final step before merging is to squash all the commits together so that the whole PR only contains 1 big commit with all your changes. We like to merge one commit at a time to keep the git history clean and navigable.

If you don't know how to do that, we have a helpful tutorial in the official documentation https://docs.godotengine.org/en/latest/community/contributing/pr_workflow.html#the-interactive-rebase

[akien-mga]

The final step before merging is to squash all the commits together so that the whole PR only contains 1 big commit with all your changes. We like to merge one commit at a time to keep the git history clean and navigable.

If you don't know how to do that, we have a helpful tutorial in the official documentation docs.godotengine.org/en/latest/community/contributing/pr_workflow.html#the-interactive-rebase

Ping @mandryskowski - are you able to do this update?

[clayjohn]

Looks great now!

[akien-mga]

Thanks!