Fix FPS counter showing initial high figure.

When the widget is created, use the current frame as reference rather than always using zero. That avoids the first FPS reading from a new widget calculating as if all frames rendered since the game started occurred in the first second.

OpenRA.Mods.Common/Widgets/Logic/PerfDebugLogic.cs

@@ -28,7 +28,7 @@ namespace OpenRA.Mods.Common.Widgets.Logic
 			perfText.IsVisible = () => Game.Settings.Debug.PerfText;
 
 			var fpsTimer = Stopwatch.StartNew();
-			var fpsReferenceFrame = 0;
+			var fpsReferenceFrame = Game.RenderFrame;
 			var fps = 0;
 			perfText.GetText = () =>
 			{