Add off-map spawn locations.

The AI isn't making use of these yet, but it's not smart enough to do so
anyway.

Would benefit from an icon to differentiate it on the map.

I'm stretching the definition of "control point" quite a bit. We might
want to put a class above `ControlPoint` for `AirSpawnLocation` to
represent types of spawn locations that can't be captured and don't have
ground objectives.

Fixes https://github.com/Khopa/dcs_liberation/issues/274

game/game.py

@@ -26,7 +26,7 @@ from .event.frontlineattack import FrontlineAttackEvent
 from .factions.faction import Faction
 from .infos.information import Information
 from .settings import Settings
-from .theater import ConflictTheater, ControlPoint
+from .theater import ConflictTheater, ControlPoint, OffMapSpawn
 from .weather import Conditions, TimeOfDay
 
 COMMISION_UNIT_VARIETY = 4
@@ -289,6 +289,9 @@ class Game:
         if len(potential_cp_armor) == 0:
             potential_cp_armor = self.theater.enemy_points()
 
+        potential_cp_armor = [p for p in potential_cp_armor if
+                              not isinstance(p, OffMapSpawn)]
+
         i = 0
         potential_units = db.FACTIONS[self.enemy_name].frontline_units