Refactor per-player shrouds & fix shellmap shroud.

OpenRA.Game/Player.cs

@@ -92,5 +92,10 @@ namespace OpenRA
 		}
 
 		public Dictionary<Player, Stance> Stances = new Dictionary<Player, Stance>();
+		public bool IsAlliedWith(Player p)
+		{
+			// Observers are considered as allies
+			return p == null || Stances[p] == Stance.Ally;
+		}
 	}
 }