glyphWidth(start, end) {
    if (end === undefined) end = start + 1;
    if (typeof this.advanceWidths === 'number')
      return Math.abs(start - end) * this.advanceWidths;
    return this.advanceWidths.slice(start, end).reduce((a, b) => a + b);
  }
Total width of glyphs by the given index range.
@param { number } start  Index of the starting glyph
@param { number } end    Index of the ending glyph
@returns { number } Total width