fixed mapper limitation creating 3x3 grids when searching for mineable locations

Fix does a lot of duplicated checks as the area gets bigger.

If you can suggest a clean way to get a spiral or an optimization that's actually worth the code complexity, let me know.

Basedefenders start to run around a bit weird as it gets bigger (I think things get so far away that other basedefenders start planting mines where the first one was headed). Wouldn't consider that a mapper issue, though. Dissenting opinion welcome.