avformat/mov: Autodetect mp3s which need parsing

mp3 packets all have the same duration and number of samples
if their duration indicated in the container varies then thats an
indication that they are not 1 mp3 packet each.
If this autodetection fails for some case then please contact us
and provide a testcase.

Fixes Ticket4938

libavformat/mov.c

@@ -3105,6 +3105,13 @@ static int mov_read_trak(MOVContext *c, AVIOContext *pb, MOVAtom atom)
         break;
     }
 
+    // If the duration of the mp3 packets is not constant, then they could need a parser
+    if (st->codec->codec_id == AV_CODEC_ID_MP3
+        && sc->stts_count > 3
+        && sc->stts_count*10 > st->nb_frames
+        && sc->time_scale == st->codec->sample_rate) {
+            st->need_parsing = AVSTREAM_PARSE_FULL;
+    }
     /* Do not need those anymore. */
     av_freep(&sc->chunk_offsets);
     av_freep(&sc->stsc_data);