Locked frame times; SAM, GUN, AGUN work again.

- all frames are 40ms long. (except something in the sidebar, which should really be PlayFetchIndex anyway)
    - SAM, GUN, and AGUN no longer crash the game when built. (Turreted used Mobile, which those buildings don't have)

OpenRa.Game/Traits/Turreted.cs

@@ -14,12 +14,9 @@ namespace OpenRa.Game.Traits
 		{
 		}
 
-		public void Tick( Actor self, Game game, int dt )
+		public void Tick( Actor self, Game game )
 		{
-			// TODO: desiredFacing should follow the base unit's facing only when not in combat.
-			// also, we want to be able to use this for GUN; avoid referencing Mobile.
-			var df = desiredFacing ?? self.traits.Get<Mobile>().facing;
-
+			var df = desiredFacing ?? ( self.traits.Contains<Mobile>() ? self.traits.Get<Mobile>().facing : turretFacing );
 			Util.TickFacing( ref turretFacing, df, self.unitInfo.ROT );
 		}
 	}