Perform widget rendering, but do not print anything.

def _render(self, content, **settings):
        """
        Perform widget rendering, but do not print anything.
        """
        lines = self._render_content(content, **settings)

        result = []
        s = self._es_margin(settings)
        for l in lines:
            result.append(self.fmt_margin(l, **s))

        return result