Forward+/Mobile: Missing sky cube reflections in shaders with specific `Environment` settings

[patwork]

Tested versions

v4.3.stable.official.77dcf97d8

System information

OpenGL API 4.2 (Core Profile) Mesa 23.2.1-1ubuntu3.1~22.04.2 - Compatibility - Using Device: Intel - Mesa Intel(R) HD Graphics 4000 (IVB GT2)

Tested also on Windows 10 with nVidia GTX 960.

Issue description

This bug is similar to issue #53817, but that case it refers to the situation when Reflected Light does not work without Sky.

Here the situation is different, Sky is generated and works, but in special cases of Environment settings, reflections in the materials stop displaying the sky. They are not black (they look different than in Disabled mode). A reflection of "something" appears on the objects, but it is not the sky.

In addition, the same settings work in Compatibility mode.

when it doesn't work

• Background == Clear Color
• Ambient Light == Background

This is how it looks with Reflected Light == Sky

For comparison Reflected Light == Background (this is because there is no Sky, same effect with Disabled)

Similar effect with Background == Custom Color

when it does work

• Background == Clear Color
• Ambient Light == Disabled ⚠️
• Reflected Light == Sky

With Ambient Light == Color

With Background == Sky and Ambient Light == Background

With Background == Clear Color and Ambient Light == Sky

Steps to reproduce

Open MRP and fiddle with Environment settings in test_scene_01.tscn.

Good first test is to change Ambient Light between Background and Disabled.

Minimal reproduction project (MRP)

sky-issues-mrp.zip

[Mingxi-Z]

Hi, this is my first contribution to Godot and I'm excited to help resolve this issue.
One can find the explanation in the pr above.

I've tested the changes on macOS (Apple M2 Max), and the reflections now render the reflected sky correctly, as shown below:

I'm new to contributing here, so any reviews or suggestions on my change would be greatly appreciated. Thanks!