This requires rendering admin boundaries based on the individual
lines, rather than the entire polygons.

* This fixes gravitystorm#621
* This possibly impacts solutions to gravitystorm#723, #2234m and gravitystorm#2663.

* Admin rendering based on lines instead of relatinos does not work (lack of
  data in e.g. Poland, and ugly rendering of dashes in case of short sections).
  Instead, this renders maritime borders based on lines, on top of the full
  border based on the polygon.
* Fix error on high zoom levels.
* Render maritime border slightly darker.