code: func OffscreenWindowNew() (*OffscreenWindow, error) {
  	c := C.gtk_offscreen_window_new()
  	if c == nil {
  		return nil, nilPtrErr
  	}
  	return wrapOffscreenWindow(glib.Take(unsafe.Pointer(c))), nil
  }
docs: // OffscreenWindowNew is a wrapper around gtk_offscreen_window_new().