Dolby AC-4 Support

It would be nice to support the detection, muxing and demuxing of Dolby AC-4 audio streams (in MP4 and MKV).

Information: https://dolbyac4.com/
Specs: https://www.dolby.com/us/en/technologies/ac-4/Next-Generation-Entertainment-Services.pdf
Sample files: http://testassets.dashif.org/#testcase/details/586fb3879ae9045678eab64f

MediaInfo 19.09 added support for AC-4, so it would be nice to have support in FFmpeg as well.
https://mediaarea.net/MediaInfo/ChangeLog