Prevent leaking of hardware cursors.

Added the lacking finialization machinery, ensured disposal under exceptional circumstances and also ensure the game calls dispose on old cursors before creating new ones.

OpenRA.Game/Game.cs

@@ -309,6 +309,9 @@ namespace OpenRA
 			using (new PerfTimer("LoadMaps"))
 				ModData.MapCache.LoadMaps();
 
+			if (Cursor != null)
+				Cursor.Dispose();
+
 			if (Settings.Graphics.HardwareCursors)
 			{
 				try