Apply overflow, justification and padding to a row.  Returns lines
        (plural) of rendered text for the row.

def format_row(self, row):
        """ Apply overflow, justification and padding to a row.  Returns lines
        (plural) of rendered text for the row. """
        assert all(isinstance(x, VTMLBuffer) for x in row)
        raw = (fn(x) for x, fn in zip(row, self.formatters))
        for line in itertools.zip_longest(*raw):
            line = list(line)
            for i, col in enumerate(line):
                if col is None:
                    line[i] = self._get_blank_cell(i)
            yield line