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.

0 files changed, 0 insertions, 0 deletions