Scale the image size.

def scale_image(self):
        """Scale the image size."""
        new_width = int(self.figcanvas.fwidth *
                        self._scalestep ** self._scalefactor)
        new_height = int(self.figcanvas.fheight *
                         self._scalestep ** self._scalefactor)
        self.figcanvas.setFixedSize(new_width, new_height)