[FFmpeg-user] Enable all audio channels

[MI]

I noticed this from Carl Eugen Hoyos in another thread ("Sony XAVC transcoding").

> (Note that you would have to add an option to
> make all tracks default in any case, imo we should not
> change the current default case. The option does not
> exist yet, mostly because I still don't understand
> the use case completely.)

If I understand correctly, there is no option to enable (mark as "default") all audio 
tracks simultaneously. This can be done in QT Pro by clicking the "default" checkbox 
for every tracks.

The frequent use case in professional post-production for this is for delivering 
Prores Quicktimes with all existing audio tracks. These are never used directly for 
playback, but for re-encoding to DCP, DVD, Bluray, VOD, etc. with various tools.

The common case I encounter is to have a single language in the .mov, as 8 individual 
mono tracks of the final audio mixes: Lt, Rt for stereo, and L, R, C, Lfe, Ls, Rs for 
5.1 (in that order).

A related question would be how to label these single mono tracks? I think ffmpeg 
does have the ability to label 1:0 as "Lt", 2:0 as "Rt", etc. but I'm not sure how 
this would be done, and I think the names ffmpeg uses are actually different.