Reduce Map.Contains(CPos) cost in legacy mods

If a mod uses rectangular maps and no height levels,
checking if the CPos is within Bounds
should be enough and cheaper than the whole ToMPos
conversion and checks.

OpenRA.Game/Map/Map.cs

@@ -752,6 +752,16 @@ public bool Contains(CPos cell)
 			if (Grid.Type == MapGridType.RectangularIsometric && cell.X < cell.Y)
 				return false;
 
+			// If the mod uses flat & rectangular maps, ToMPos and Contains(MPos) create unnecessary cost.
+			// Just check if CPos is within map bounds.
+			if (Grid.MaximumTerrainHeight == 0 && Grid.Type == MapGridType.Rectangular)
+			{
+				if (!Bounds.Contains(cell.X, cell.Y))
+					return false;
+
+				return true;
+			}
+
 			return Contains(cell.ToMPos(this));
 		}