Change how video timing is done.

Change how video timing is done.

This commit changes the approach with video timing.  Previously,
we would (more-or-less) try to use every video frame from the content
in the output, hoping that they come at a constant frame rate.

This is not always the case, however.  Here we preserve the PTS
of video frames, and then when one arrives we output whatever
DCP video frames we can (at the regular DCP frame rate).

Hopefully this will solve a range of sync problems, but it
could also introduce new ones.