Returns code points of characters included in given font files.

def _getFontChars(self, font_files):
        """
        Returns code points of characters included in given font files.
        """
        code_points = set()
        for font_file in font_files:
            face = ft.Face(font_file)
            charcode, agindex = face.get_first_char()
            while agindex != 0:
                code_points.add(charcode)
                charcode, agindex = face.get_next_char(charcode, agindex)
        return sorted(code_points)