Significantly improve actor placement and removal speeds

OpenRA.Game/Map/CellCoordsRegion.cs

@@ -9,6 +9,7 @@
  */
 #endregion
 
+using System;
 using System.Collections;
 using System.Collections.Generic;
 
@@ -89,6 +90,31 @@ namespace OpenRA
 			return GetEnumerator();
 		}
 
+		/// <summary>Returns the minimal region that covers at least the specified cells.</summary>
+		public static CellCoordsRegion BoundingRegion(IReadOnlyCollection<CPos> cells)
+		{
+			if (cells == null || cells.Count == 0)
+				throw new ArgumentException("cells must not be null or empty.", nameof(cells));
+
+			var minX = int.MaxValue;
+			var minY = int.MaxValue;
+			var maxX = int.MinValue;
+			var maxY = int.MinValue;
+			foreach (var cell in cells)
+			{
+				if (minX > cell.X)
+					minX = cell.X;
+				if (maxX < cell.X)
+					maxX = cell.X;
+				if (minY > cell.Y)
+					minY = cell.Y;
+				if (maxY < cell.Y)
+					maxY = cell.Y;
+			}
+
+			return new CellCoordsRegion(new CPos(minX, minY), new CPos(maxX, maxY));
+		}
+
 		public CPos TopLeft { get; }
 		public CPos BottomRight { get; }
 	}