Make refraction mixing based on water depth instead of screen depth

Makes refraction fading into the water color relative to the water surface instead of the camera. This fixes looking at the water from extremely high up, also looking at distant shores.

A small amount of the old fading is mixed in so that the water level remains somewhat easy to see even if it's not reflecting anything.

Also changes the sun's scattering color because without doing so the sun is a radioactive green.