Incompatible behavior to use the Seek function of the s2.ReadSeeker with s2 Index when using whence=io.SeekEnd

• Version: v1.15.2
• Issue description:
  • filesize, err = rs.Seek(0, io.SeekEnd), expect to get the uncompressed file size, but currently return 0
  • pos, err = rs.Seek(-10, io.SeekEnd), expect to get position at last 10-bytes from end of uncompressed file, but currently return 10