As the removed comment says, not copying this field may cause problems
on older libav* releases. See also commit 5f7de399.
Remove this, as newer FFmpeg releases are available. As of this commit,
use of mpv with FFmpeg 2.5.x and below, or Libav 11 and below is not
recommended, and may lead to random video decoding issues. (Although the
failure cases are apparently somewhat obscure.)