lavf/mov: improve `tref/chap` chapter handling

3 parts:
- Supports multiple chapter streams
- Exports regular text chapter streams as opaque data. This prevents consumers
  from showing chapters as if they were regular subtitle streams.
- Exports video chapter streams as thumbnails, and provides the first one as
  an attached_pic.

libavformat/isom.h

@@ -210,7 +210,8 @@ typedef struct MOVContext {
     unsigned trex_count;
     int itunes_metadata;  ///< metadata are itunes style
     int handbrake_version;
-    int chapter_track;
+    int *chapter_tracks;
+    unsigned int nb_chapter_tracks;
     int use_absolute_path;
     int ignore_editlist;
     int ignore_chapters;

libavformat/mov.c

@@ -3992,7 +3992,20 @@ static int mov_read_tfhd(MOVContext *c, AVIOContext *pb, MOVAtom atom)
 
 static int mov_read_chap(MOVContext *c, AVIOContext *pb, MOVAtom atom)
 {
-    c->chapter_track = avio_rb32(pb);
+    unsigned i, num;
+    void *new_tracks;
+
+    num = atom.size / 4;
+    if (!(new_tracks = av_malloc_array(num, sizeof(int))))
+        return AVERROR(ENOMEM);
+
+    av_free(c->chapter_tracks);
+    c->chapter_tracks = new_tracks;
+    c->nb_chapter_tracks = num;
+
+    for (i = 0; i < num && !pb->eof_reached; i++)
+        c->chapter_tracks[i] = avio_rb32(pb);
+
     return 0;
 }
 
@@ -5055,25 +5068,50 @@ static int mov_probe(AVProbeData *p)
 static void mov_read_chapters(AVFormatContext *s)
 {
     MOVContext *mov = s->priv_data;
-    AVStream *st = NULL;
+    AVStream *st;
     MOVStreamContext *sc;
     int64_t cur_pos;
-    int i;
+    int i, j;
+    int chapter_track;
 
+    for (j = 0; j < mov->nb_chapter_tracks; j++) {
+    chapter_track = mov->chapter_tracks[j];
+    st = NULL;
     for (i = 0; i < s->nb_streams; i++)
-        if (s->streams[i]->id == mov->chapter_track) {
+        if (s->streams[i]->id == chapter_track) {
             st = s->streams[i];
             break;
         }
     if (!st) {
         av_log(s, AV_LOG_ERROR, "Referenced QT chapter track not found\n");
-        return;
+        continue;
     }
 
-    st->discard = AVDISCARD_ALL;
     sc = st->priv_data;
     cur_pos = avio_tell(sc->pb);
 
+    if (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) {
+        st->disposition |= AV_DISPOSITION_ATTACHED_PIC | AV_DISPOSITION_TIMED_THUMBNAILS;
+        if (st->nb_index_entries) {
+            // Retrieve the first frame, if possible
+            AVPacket pkt;
+            AVIndexEntry *sample = &st->index_entries[0];
+            if (avio_seek(sc->pb, sample->pos, SEEK_SET) != sample->pos) {
+                av_log(s, AV_LOG_ERROR, "Failed to retrieve first frame\n");
+                goto finish;
+            }
+
+            if (av_get_packet(sc->pb, &pkt, sample->size) < 0)
+                goto finish;
+
+            st->attached_pic              = pkt;
+            st->attached_pic.stream_index = st->index;
+            st->attached_pic.flags       |= AV_PKT_FLAG_KEY;
+        }
+    } else {
+    st->codecpar->codec_type = AVMEDIA_TYPE_DATA;
+    st->codecpar->codec_id = AV_CODEC_ID_BIN_DATA;
+    st->discard = AVDISCARD_ALL;
     for (i = 0; i < st->nb_index_entries; i++) {
         AVIndexEntry *sample = &st->index_entries[i];
         int64_t end = i+1 < st->nb_index_entries ? st->index_entries[i+1].timestamp : st->duration;
@@ -5122,8 +5160,10 @@ static void mov_read_chapters(AVFormatContext *s)
         avpriv_new_chapter(s, i, st->time_base, sample->timestamp, end, title);
         av_freep(&title);
     }
+    }
 finish:
     avio_seek(sc->pb, cur_pos, SEEK_SET);
+    }
 }
 
 static int parse_timecode_in_framenum_format(AVFormatContext *s, AVStream *st,
@@ -5446,7 +5486,7 @@ static int mov_read_header(AVFormatContext *s)
     av_log(mov->fc, AV_LOG_TRACE, "on_parse_exit_offset=%"PRId64"\n", avio_tell(pb));
 
     if (pb->seekable) {
-        if (mov->chapter_track > 0 && !mov->ignore_chapters)
+        if (mov->nb_chapter_tracks > 0 && !mov->ignore_chapters)
             mov_read_chapters(s);
         for (i = 0; i < s->nb_streams; i++)
             if (s->streams[i]->codecpar->codec_tag == AV_RL32("tmcd")) {