Fix cargo dest overlay not being updated after a zoom change.

src/main_gui.cpp

@@ -184,6 +184,9 @@ bool DoZoomInOutWindow(ZoomStateChange how, Window *w)
 	}
 	/* Update the windows that have zoom-buttons to perhaps disable their buttons */
 	w->InvalidateData();
+	if (how != ZOOM_NONE) {
+		RebuildViewportOverlay(w);
+	}
 	return true;
 }