Separate sky luminance and brightness calculations for consistent fog

[Namey5]

Luminance + user-controlled brightness multipliers were combined into a single push-constant for application in the Sky shader - this had the side effect of accumulated fog also being controlled by the environment brightness value (and not just renderer luminance).

Before (Forward+, volumetric fog):

After (Forward+, volumetric fog):

After (Forward+, distance fog):

After (Mobile, distance fog - just to show luminance still working):

This is my first time delving into actual render code in Godot, so I have implemented this recommendation in the simplest way possible - by keeping configuration the same and literally just splitting the two multipliers into separate push constants.

Fixes #98640

[clayjohn]

Looks great. This matches how it works in the Compatibility renderer, so now everything is consistent too, which is nice.

[Repiteo]

Thanks!