For Duke Nukem and its direct offspring (Nam, WW2GI and Redneck Rampage) the ramp is not a linear fade from 0 to 1, it needs to be a little darker than that.
Unfortunately the proper factor needed here must be set manually, this cannot really be calculated from the lookup tables.
The only compatibility mode feature left is the main drawer function using glBegin/glEnd but changing that is not as urgent as the rest.
This also cleans up the fog application and adds the exponential fog mode again that somehow got lost over time.