Added radiance when using clear color

[clayjohn]

Before when using a background other than sky, when METALLIC was greater than zero the mesh rendered black. Now, it takes into account background color when not using an environment map (not using sky).

before

after

Note: if our PBR implementation was perfectly energing-preserving, this sphere should be perfect white, but it is not. I'm going to look into that for a further PR, we should be able to approximate multi scattering and make our specular function closer to energy preserving.

Fixes: #27888

[clayjohn]

I just added another change to this. I moved the computation of the environment brdf to the beginning where the radiance map is read from. A term that should have only applied to the environment was being used to scale the entire brdf, resulting in much darker rough metals.

before

after

And now our brdf is energy conserving again!

[clayjohn]

@akien-mga i can split this up into two PRs. The background color stuff should not break compatibility. And the PBR correctness part can wait for 4.0 if we are worried about breaking projects. Although the difference is very subtle.

[reduz]

This should be left as-is I think. The current implementation is very confusing and this will most likely make it worse (It will break compatibility and users will be clueless about how to fix it, since previous behavior will not be possible anymore). For Godot 4, my plan is to separate the sources for indirect irradiance and reflections, so it would be more obvious what's going on.

[clayjohn]

@reduz What do you mean? Previous behaviour is that as soon as Metallic goes above 0.0all meshes render completely black. There is no way that users rely on that behaviour. Any user would report that as a bug.

Plus there are many issues that have been opened related to this. see: #27322 and #21680

[akien-mga]

As discussed on IRC today, there are some concerns on the impact this may have on existing projects. Yet it's also a clear bugfix, so the plan is to merge it, ask people to test the master branch on their projects, and assess anything that could be consider problematic in those projects.

Some changes needed based on today's review before merging, though.

[clayjohn]

As per meeting discussion I will move the environment scaling to 1628 in GLES2 and to 2025 in GLES3

[akien-mga]

Thanks!