movenc: add fallback audio track tref support

This feature allows making associations between audio tracks
that apple players recognize.  E.g. when an ac3 track has a
tref that points to an aac track, devices that don't support
ac3 will automatically fall back to the aac track.

Apple used to *guess* these associations, but new products
(AppleTV 4) no longer guess and this association can only
be made explicitly now using the "fall" tref.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavformat/movenc.c

@@ -2285,10 +2285,21 @@ static int mov_write_moov_tag(AVIOContext *pb, MOVMuxContext *mov,
             mov->tracks[i].tref_id  = mov->tracks[mov->chapter_track].track_id;
         }
     for (i = 0; i < mov->nb_streams; i++) {
-        if (mov->tracks[i].tag == MKTAG('r','t','p',' ')) {
-            mov->tracks[i].tref_tag = MKTAG('h','i','n','t');
-            mov->tracks[i].tref_id =
-                mov->tracks[mov->tracks[i].src_track].track_id;
+        MOVTrack *track = &mov->tracks[i];
+        if (track->tag == MKTAG('r','t','p',' ')) {
+            track->tref_tag = MKTAG('h','i','n','t');
+            track->tref_id = mov->tracks[track->src_track].track_id;
+        } else if (track->enc->codec_type == AVMEDIA_TYPE_AUDIO) {
+            int * fallback, size;
+            fallback = (int*)av_stream_get_side_data(track->st,
+                                                     AV_PKT_DATA_FALLBACK_TRACK,
+                                                     &size);
+            if (fallback != NULL && size == sizeof(int)) {
+                if (*fallback >= 0 && *fallback < mov->nb_streams) {
+                    track->tref_tag = MKTAG('f','a','l','l');
+                    track->tref_id = mov->tracks[*fallback].track_id;
+                }
+            }
         }
     }