Fog enhancements

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The current 1 color fog is ok but we could have something new. It's not that 
difficult since we have only 3 .shader files.

I want at least 2 colors for fog: 1st under the sun and 2nd on the opposite 
side (use cam dir dot? with sun dir to blend between them). 
It's clear that those are needed with sunset, dawn, and such skyboxes.
Could use alpha from color to adjust eg. intensity, may look nice.
Maybe a 3rd color for upper sky ? Or even a small 3d texture?
Here's a shot from 2006, can't be difficult: http://imgur.com/dGs4b8i

And other thing, height fog (added to the base fog). So we could have fog near 
terrain or closing some height value.
I think there should be some papers on fog, if you know them or have other 
ideas, post links here.

Original issue reported on code.google.com by Cry...@gmail.com on 23 Mar 2013 at 12:58

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Original comment by Cry...@gmail.com on 31 Mar 2013 at 11:01

• Added labels: Progress-60
• Removed labels: Progress-30

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Ok done.
We have now 2 colors for normal fog - under sun and away (opposite dir) taken 
from camera xz dir.
And height fog.
All have colors with alpha. Normally 1, but less can be used to make not full 
density.

Original comment by Cry...@gmail.com on 3 Apr 2013 at 4:36

• Added labels: Progress-90
• Removed labels: Progress-60

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Original comment by Cry...@gmail.com on 3 Apr 2013 at 6:29

• Changed state: Fixed