Allow Attack activities to target FrozenActors directly.

Removing the legacy FrozenActor to Actor workaround
fixes a number of long-standing bugs.

This also prevents units from losing their target when
it transforms into a different actor type.

OpenRA.Mods.Common/Activities/Air/FlyAttack.cs

@@ -18,10 +18,10 @@ namespace OpenRA.Mods.Common.Activities
 {
 	public class FlyAttack : Activity
 	{
-		readonly Target target;
 		readonly Aircraft aircraft;
 		readonly AttackAircraft attackAircraft;
 		readonly Rearmable rearmable;
+		Target target;
 
 		int ticksUntilTurn;
 
@@ -43,6 +43,8 @@ namespace OpenRA.Mods.Common.Activities
 				return NextActivity;
 			}
 
+			target = target.Recalculate(self.Owner);
+
 			if (!target.IsValidFor(self))
 				return NextActivity;