Start filtering. It simply create a new canvas (the old one is saved).

def start_filter(self):
        """
        Start filtering. It simply create a new canvas (the old one is saved).
        """
        self._filter_renderers.append(self._renderer)
        self._renderer = _RendererAgg(int(self.width), int(self.height),
                                      self.dpi)
        self._update_methods()