Fog color is too dark

[jjspace]

While working on #12248 I noticed the fog color overall seems too dark compared to other platforms like Google Earth. I would expect the colors to more closely match the light white/yellow at the base of the atmosphere.

This gets even worse/more obvious if you crank up the density

Looking back at some older fog related issues it seems like it wasn't always this way so hopefully it's an easy fix that was just missed somewhere along the way

From #8454

or

[ggetz]

I think I have a fix for this started in https://github.com/CesiumGS/cesium/compare/atmopshere-scattering-fx?expand=1

Basically ground atmosphere scattering is not using the same amount of light color as the sky, plus the alpha value was not being correctly computed.

[jjspace]

As discussed in #12248 when the issues with the fog darkness are fixed we should increase the default visualDensityScalar so fog is more visually impactful.