Fix swap buffer & rendering skipped when there's no room

This fix will allow the game state to have no room, and any GUIs which
have no room will still be rendered (and bg color will be colored
black). This should fix #160

Source/Engine/AGS.Engine/Graphics/Logic/AGSRendererLoop.cs

@@ -49,15 +49,15 @@ public class AGSRendererLoop : IRendererLoop
 
 		public bool Tick ()
 		{
-            if (_gameState.Room == null) return false;
-			IRoom room = _gameState.Room;
             _glUtils.RefreshViewport(_game.Settings, _gameWindow, _gameState.Viewport);
 
-			switch (_roomTransitions.State)
+			var transitionState = _roomTransitions.State;
+            if (_gameState.Room == null) transitionState = RoomTransitionState.NotInTransition; //If there's no room, then room transition state is meaningless -> we'll interpret as not in transition, which will render the viewports, without a room (so GUIs will still be rendered).
+			switch (transitionState)
 			{
 				case RoomTransitionState.NotInTransition:
 					activateShader();
-					renderRoom();
+					renderAllViewports();
 					break;
 				case RoomTransitionState.BeforeLeavingRoom:
                     if (_roomTransitions.Transition == null)
@@ -121,12 +121,12 @@ private IFrameBuffer renderToBuffer()
             TypedParameter sizeParam = new TypedParameter(typeof(Size), _game.Settings.WindowSize);
             IFrameBuffer frameBuffer = _resolver.Container.Resolve<IFrameBuffer>(sizeParam);
 			frameBuffer.Begin();
-			renderRoom();
+			renderAllViewports();
 			frameBuffer.End();
 			return frameBuffer;
 		}
 
-		private void renderRoom()
+        private void renderAllViewports()
 		{
             renderViewport(_gameState.Viewport);
             try