As packet duration is not stored inherently in MPEG4 containers,
subtitles have their duration expressed by storing an additional
empty packet with a pts matching the desired end time of the real
subtitle. Additionally, it is generally expected that all streams
start at time = 0, so an empty packet needs to be inserted at the
beginning of the stream, before the first real subtitle.

Unfortunately, ffmpeg lacks a proper way to express that a subtitle
might map to multiple packets, so the muxer is the only place we
can handle this.
Signed-off-by: Philip Langdale <philipl@overt.org>