Add a graphical option to draw a solid color border around turret coverage

[jmillikin]

I'm having a difficult time viewing turret coverage on Gleba due to the color scheme. The purple terrain is close enough to red that the shading isn't distinguishable, and the green shaded by red is very close to the color of the brown "highlands".

Would it be possible to get a graphical option to make the turret coverage have a solid color border? Even just a few pixels thick would be fine.