lavf/movenc: Write 'dby1' minor brand if Dolby content is being muxed to MP4

This is as per:
   * mp4ra: http://mp4ra.org/#/brands
   * Dolby Vision muxing spec (which is public):
       https://professional.dolby.com/siteassets/content-creation/dolby-vision-for-content-creators/dolby_vision_bitstreams_within_the_iso_base_media_file_format_dec2017.pdf

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

libavformat/movenc.c

@@ -4991,7 +4991,7 @@ static int mov_write_ftyp_tag(AVIOContext *pb, AVFormatContext *s)
 {
     MOVMuxContext *mov = s->priv_data;
     int64_t pos = avio_tell(pb);
-    int has_h264 = 0, has_av1 = 0, has_video = 0;
+    int has_h264 = 0, has_av1 = 0, has_video = 0, has_dolby = 0;
     int i;
 
     for (i = 0; i < s->nb_streams; i++) {
@@ -5004,6 +5004,11 @@ static int mov_write_ftyp_tag(AVIOContext *pb, AVFormatContext *s)
             has_h264 = 1;
         if (st->codecpar->codec_id == AV_CODEC_ID_AV1)
             has_av1 = 1;
+        if (st->codecpar->codec_id == AV_CODEC_ID_AC3 ||
+            st->codecpar->codec_id == AV_CODEC_ID_EAC3 ||
+            st->codecpar->codec_id == AV_CODEC_ID_TRUEHD ||
+            av_stream_get_side_data(st, AV_PKT_DATA_DOVI_CONF, NULL))
+            has_dolby = 1;
     }
 
     avio_wb32(pb, 0); /* size */
@@ -5029,6 +5034,8 @@ static int mov_write_ftyp_tag(AVIOContext *pb, AVFormatContext *s)
                 ffio_wfourcc(pb, "iso6");
             if (has_av1)
                 ffio_wfourcc(pb, "av01");
+            if (has_dolby)
+                ffio_wfourcc(pb, "dby1");
         } else {
             if (mov->flags & FF_MOV_FLAG_FRAGMENT)
                 ffio_wfourcc(pb, "iso6");

tests/ref/fate/copy-trac3074

@@ -1,5 +1,5 @@
-da6122873fb83ce4340cf5d0ab8d475e *tests/data/fate/copy-trac3074.mp4
-334012 tests/data/fate/copy-trac3074.mp4
+452d91e7c6889b787717fef25b6fce43 *tests/data/fate/copy-trac3074.mp4
+334016 tests/data/fate/copy-trac3074.mp4
 #tb 0: 1/48000
 #media_type 0: audio
 #codec_id 0: eac3