Codechange: Pass Viewport by reference.

This means we do not have to care what type of pointer is used.

src/viewport_gui.cpp

@@ -140,7 +140,7 @@ public:
 	{
 		if (!gui_scope) return;
 		/* Only handle zoom message if intended for us (msg ZOOM_IN/ZOOM_OUT) */
-		HandleZoomMessage(this, this->viewport, WID_EV_ZOOM_IN, WID_EV_ZOOM_OUT);
+		HandleZoomMessage(this, *this->viewport, WID_EV_ZOOM_IN, WID_EV_ZOOM_OUT);
 	}
 };