avformat/movenc: allow writing out channel count in MP4 and 3GP

ISOBMFF (14496-12) made this field ('channelcount') in the
AudioSampleEntry structure non-template¹ somewhere before the
release of the 2022 edition. As for ETSI TS 126 244 AKA 3GPP
file format (V16.1.0, 2020-10), it does not seem contain any
references limiting the channelcount entry in AudioSampleEntry
or in its own definition of EVSSampleEntry.

fate-mov-mp4-chapters test had to be adjusted as it output a
mono vorbis stream, which would now be properly marked as such
in the container.

1: As per 14496-12:
   Fields shown as “template” in the box descriptions are fields
   which are coded with a default value unless a derived
   specification defines their use and permits writers to use
   other values than the default.

libavformat/movenc.c

@@ -1241,13 +1241,7 @@ static int mov_write_audio_tag(AVFormatContext *s, AVIOContext *pb, MOVMuxContex
                 avio_wb16(pb, 16);
             avio_wb16(pb, track->audio_vbr ? -2 : 0); /* compression ID */
         } else { /* reserved for mp4/3gp */
-            if (track->par->codec_id == AV_CODEC_ID_FLAC ||
-                track->par->codec_id == AV_CODEC_ID_ALAC ||
-                track->par->codec_id == AV_CODEC_ID_OPUS) {
-                avio_wb16(pb, track->par->ch_layout.nb_channels);
-            } else {
-                avio_wb16(pb, 2);
-            }
+            avio_wb16(pb, track->par->ch_layout.nb_channels);
             if (track->par->codec_id == AV_CODEC_ID_FLAC ||
                 track->par->codec_id == AV_CODEC_ID_ALAC) {
                 avio_wb16(pb, track->par->bits_per_raw_sample);

tests/ref/fate/mov-mp4-chapters

@@ -1,4 +1,4 @@
-1fd844c2f5bf77c3344e88e30ad994e1 *tests/data/fate/mov-mp4-chapters.mp4
+7b6aaa99c86fa1f5abfc9f242abcfffa *tests/data/fate/mov-mp4-chapters.mp4
 111248 tests/data/fate/mov-mp4-chapters.mp4
 #extradata 0:     3469, 0xc6769ddc
 #tb 0: 1/44100