demux: force reading packets again after seeks

in->eof is used as an indicator whether reading packets still makes
sense. (Without this, the prefetcher would obviously burn CPU by
retrying reading even though everything has been read.)

This was not reset properly after seeks were performed. It led to
getting stuck in at least one corner case: when enabling a track, the
demuxer would seek backwards to get new packets from the current
playback position ("refresh seeks"). But if playback was paused, and EOF
was previously reached, it would not try to read packers again due to
in->eof being false. There was not anything else that would make it
retry reading, so it was stuck in a weird underrun/buffering state.

Fixes: #6986

demux/demux.c

@@ -2347,7 +2347,7 @@ static void execute_seek(struct demux_internal *in)
 {
     int flags = in->seek_flags;
     double pts = in->seek_pts;
-    in->last_eof = false;
+    in->last_eof = in->eof = false;
     in->seeking = false;
     in->seeking_in_progress = pts;
     in->demux_ts = MP_NOPTS_VALUE;