demux_timeline: warn if streams are invisible

ytdl_hook.lua can do this with all_formats and when delay_open is used,
and if the source stream actually contains both audio and video. In this
case, it might accidentally hide a media type completely, or waste
bandwidth (if the stream has true interleaved audio/video). So it's
important to warn.

demux/demux_timeline.c

@@ -116,6 +116,7 @@ static void associate_streams(struct demuxer *demuxer,
     for (int n = 0; n < num_streams; n++) {
         struct sh_stream *sh = demux_get_stream(seg->d, n);
         struct virtual_stream *other = NULL;
+
         for (int i = 0; i < src->num_streams; i++) {
             struct virtual_stream *vs = src->streams[i];
 
@@ -134,6 +135,11 @@ static void associate_streams(struct demuxer *demuxer,
                 other = vs;
         }
 
+        if (!other) {
+            MP_WARN(demuxer, "Source stream %d (%s) unused and hidden.\n",
+                    n, stream_type_name(sh->type));
+        }
+
         MP_TARRAY_APPEND(seg, seg->stream_map, seg->num_stream_map, other);
     }
 }