Three state fog of war

[jkufner]

Clouds are now two state: transparent (some unit can see there) or opaque (no unit can see there).

They should be three state: transparent, opaque, and explored (terrain is visible).

When a unit visits some area, clouds over once seen tiles should never go to fully opaque. They should be 50% transparent, so the tile under is visible, but units should be hidden, because no unit can see what is happening on the tile. And also buildings should be rendered in neutral colors.