Adds the ability for SO to distribute a file with the
client, which is drawn as a backdrop in the map pane.
The backdrop can be toggled on and off with
the "overlay" option in the map menu.
File format is this:
LINE is a drawing primitive, only one supported now
x y is the coordinates of each vertex on the line, in
the same coordinate space as planets.
The file is data/mapoverlay.txt