In older versions of ffmpeg (incl. ffmpeg version 3.4.8-0ubuntu0.2 ),
using the end-time (-to) causes a syntax error, so we need to switch
to using the duration.
The library mis-match on ubuntu 18.04 seems impossible to fix, so
revert to using the system binaries.
This makes including a subtitle stream impossible for now, but moving
to the concat *filter* had made that pretty much impossible anyway,
because ffmpeg (even in HEAD) does not set the right PTS when the
input is in matroska format.
the original code used >= or <= as the test, but the code actually
implemented > or <. This causes an ffmpeg error when there is only 1
i-frame between the lower and upper bound.
-use the concat filter rather than segment+concat muxers
-currently, do not create a separate subtitle stream from closed
captions
-ffmpeg-myth-do-pending-jobs will look up recordings with a cutlist and
run the transcoder (ffmpeg-myth-cut) over each
