code: func ViewportNew(hadjustment, vadjustment *Adjustment) (*Viewport, error) {
  	c := C.gtk_viewport_new(hadjustment.native(), vadjustment.native())
  	if c == nil {
  		return nil, nilPtrErr
  	}
  	return wrapViewport(glib.Take(unsafe.Pointer(c))), nil
  }
docs: // ViewportNew() is a wrapper around gtk_viewport_new().