Improve game loop

Environment.TickCount was replaced with Game.RunTime that's based on
Stopwatch for increased accuracy.

OpenRA.Game/Network/OrderManager.cs

@@ -35,7 +35,7 @@ namespace OpenRA.Network
 		public int LocalFrameNumber;
 		public int FramesAhead = 0;
 
-		public int LastTickTime = Environment.TickCount;
+		public int LastTickTime = Game.RunTime;
 
 		public bool GameStarted { get { return NetFrameNumber != 0; } }
 		public IConnection Connection { get; private set; }