code: function writeSubTableHeader(writer, platform, encoding, offset) {
              writer.writeUint16(platform); // platform
              writer.writeUint16(encoding); // encoding
              writer.writeUint32(offset); // offset
              return writer;
          }
docs: 写subtableheader
  
  @param {Writer} writer Writer对象
  @param {number} platform 平台
  @param {number} encoding 编码
  @param {number} offset 偏移
  @return {Writer}