Introducing per-player shrouds.

- Each player has their own shroud and their visibility does not extend outside of the shroud. 
- Units and buildings can no longer target other units outside of their visibility. Buildings can still be targetted if they have been explored.
- GPS will provide visibility in the fog-of-war.
- Spies that infiltrate radar domes will gain their victim's exploration and reset it on all clients (if the victim does not have GPS)

OpenRA.Mods.RA/Activities/Attack.cs

@@ -54,6 +54,9 @@ namespace OpenRA.Mods.RA.Activities
 
 			if (!Target.IsValid)
 				return NextActivity;
+				
+			if (!self.Owner.HasFogVisibility() && Target.Actor != null && Target.Actor.HasTrait<Mobile>() && !self.Owner.Shroud.IsTargetable(Target.Actor))
+				return NextActivity;
 
 			if (targetable != null && !targetable.TargetableBy(Target.Actor, self))
 				return NextActivity;