code: function parseCFFHead(reader) {
              var head = {};
              head.startOffset = reader.offset;
              head.endOffset = head.startOffset + 4;
              head.formatMajor = reader.readUint8();
              head.formatMinor = reader.readUint8();
              head.size = reader.readUint8();
              head.offsetSize = reader.readUint8();
              return head;
          }
docs: 解析cff表头部
  @param  {Reader} reader 读取器
  @return {Object}        头部字段