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There is a lot of dancing about to handle potential gaps and sync
problems in the FFmpeg decoder.  It might be nicer if
\texttt{FFmpegDecoder} could just spit out video and audio with
timestamps and let the player sort it out, since the player must
already handle insertion of black and silence.

The first question would be what time unit the decoder should use to
stamp its output.  Initially we have the PTS, in some time base, and
we can convert that to seconds at the content's frame rate; this is
basically a \texttt{Time}.  So we could emit video and audio content
with \texttt{Time} stamps.

Then the player receives video frames, and can fill in gaps.

The FFmpeg decoder would still have to account for non-zero initial
PTS, as it is expected that such `dead time' is trimmed from the
source implicitly.

The snag with this is that hitherto \texttt{Time} has meant DCP time,
not time at a content's rates (before the content is potentially sped
up).  As it stands, seek takes a \texttt{Time} in the DCP and the
content class converts it to content frames.  This is then (rather
grottily) converted back to time again via the content frame rate.
All a bit grim.  Everything should probably work in time rather than
frame rates.

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