This way, when metadata is updated, we only replace the attachments
produced by the demuxer whose metadata changed.

Fixes #11976

Tested by reverting the patch in #11966 (with some fix-ups to pass the
right arguments where the code had changed).
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>
(cherry picked from commit 223a42c1ec748dd5a0b338c90c571a37483f3297)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>