Fog shaders for depth fog

[nenad-jalsovec]

It'd be nice to have per pixel control over how old school depth fog in rendered. I think engine neglected the regular fog in favor of fancier volumetric fog, forgetting how useful the basic fog actually is in many types of games.

I have a project with a spherical planet and I'm rendering atmospheric perspective using fog. The fog needs to have different parameters on the dark side of the planet. This would be easy to do with some kind of fog shader.

Without that, the only option is making coustom fog using the compositor, which is kinda complicated.

[Calinou]

Thanks for the proposal! Consolidating in #2643.