Remove sleep time when the game is inactive

Fixes jittery/slow rendering speed when the game is not the focused window.

EndlessClient/GameExecution/EndlessGame.cs

@@ -120,6 +120,7 @@ protected override void Initialize()
             IsFixedTimeStep = false;
 
             TargetElapsedTime = TimeSpan.FromMilliseconds(FixedTimeStepRepository.TICK_TIME_MS);
+            InactiveSleepTime = TimeSpan.FromMilliseconds(0);
 
             _previousKeyState = Keyboard.GetState();