Extract ViewportEdgeScrollMargin settings entry.

OpenRA.Game/Settings.cs

@@ -157,6 +157,8 @@ namespace OpenRA
 		public string Platform = "Default";
 
 		public bool ViewportEdgeScroll = true;
+		public int ViewportEdgeScrollMargin = 5;
+
 		public bool LockMouseWindow = false;
 		public MouseScrollType MiddleMouseScroll = MouseScrollType.Standard;
 		public MouseScrollType RightMouseScroll = MouseScrollType.Disabled;