docs: 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)