Rework Fog into Sky Space

[MilchRatchet]

Currently fog is done in world space centered around the player. Issue is that while that may work relative to geometry, any interactions with the sky become weird. My idea is to instead let the fog be done in sky space with density decreasing with height. The main slider would then be the base density. This should allow for foggy scenes without messing up sky contribution. Further, clouds should now vanish smoothly close to the horizon.