Introduce ScreenMap trait for caching screen-coord queries.

OpenRA.Game/Traits/Waypoint.cs

@@ -18,7 +18,7 @@ namespace OpenRA.Traits
 		public object Create( ActorInitializer init ) { return new Waypoint( init ); }
 	}
 
-	class Waypoint : IOccupySpace, ISync
+	class Waypoint : IOccupySpace, ISync, INotifyAddedToWorld, INotifyRemovedFromWorld
 	{
 		[Sync] CPos location;
 
@@ -30,5 +30,15 @@ namespace OpenRA.Traits
 		public CPos TopLeft { get { return location; } }
 		public IEnumerable<Pair<CPos, SubCell>> OccupiedCells() { yield break; }
 		public WPos CenterPosition { get { return location.CenterPosition; } }
+
+		public void AddedToWorld(Actor self)
+		{
+			self.World.ScreenMap.Add(self);
+		}
+
+		public void RemovedFromWorld(Actor self)
+		{
+			self.World.ScreenMap.Remove(self);
+		}
 	}
 }