Fix case where the is FFmpegContent with 24fps video and 44.1kHz audio

Fix case where the is FFmpegContent with 24fps video and 44.1kHz audio
and a start trim of 6724000.  With these numbers the start trim is
on an integer video frame but halfway through an audio frame.  Without
this patch the trim would be "corrected" to 6724001, causing video frames
to come out of the player at DCPTimes 0, 3999, 5999 etc.

It's possible that

Frame const position = time.frames_floor(_film->video_frame_rate());

in J2KEncoder::encode should be frames_round, which would also help
with this, but that would be a much more risky patch.