Work around transcode failures in newer ffmpeg

Sometimes ffmpeg 3.4 and 4.0 fail on transcodes with errors like
"1 frames left in the queue on closing"

Per https://trac.ffmpeg.org/ticket/6375 the workaround is to add
the param -max_muxing_queue_size with some suitably large number
which usually fixes most of them, allegedly.

Change-Id: I52e8d016f46ca489d4e388ec207355cb651e4d4b