Return list of glyphs and glyph widths of a font.

def _getCharWidths(self, xref, bfname, ext, ordering, limit, idx=0):
        """Return list of glyphs and glyph widths of a font."""
        if self.isClosed or self.isEncrypted:
            raise ValueError("operation illegal for closed / encrypted doc")

        return _fitz.Document__getCharWidths(self, xref, bfname, ext, ordering, limit, idx)