OpenGL: Fog color is inverted if density is exactly 0

[Calinou]

Related to #66456 and #66459.

Godot version

4.0.beta (53d2a9a)

System information

Fedora 36, GeForce GTX 1080 (NVIDIA 515.65.01)

Issue description

Fog color is inverted if density is exactly 0. This doesn't occur when using the Vulkan Clustered renderer.

The black sky in the screenshots below is an unrelated issue. It likely comes from a division by zero or NaN being produced by the shader – see #66459.

Very low but non-zero density

Zero density

Steps to reproduce

• Switch to the OpenGL driver in the Project Settings.
  • Make sure to start the editor with single-window mode enabled afterwards. This can be done by enabling Single Window Mode in the Editor Settings before changing the driver, or by opening the project in the editor directly with the --single-window command line argument.
• Add a WorldEnvironment node, a Camera3D node and a MeshInstance3D in front of the camera.
• Enable fog. Decrease density to 0 using the slider.

Minimal reproduction project

test_opengl_fog_sky_affect.zip