Unlike other cases where the closed captions are embedded in the
video stream as MPEG-2 userdata or H.264 SEI data, with MOV files
the captions are often found on a separate "e608" subtitle track.

Add support for playout of such files, leveraging the new ccfifo
mechanism to ensure that they are embedded into VANC at the correct
rate (since e608 packets often contain batches of multiple 608 pairs).

Note this patch includes a new file named libavdevice/ccfifo.c, which
allows the ccfifo functionality in libavfilter to be reused even if
doing shared builds.  This is the same approach used for log2_tab.c.
Signed-off-by: Devin Heitmueller <dheitmueller@ltnglobal.com>
Signed-off-by: Limin Wang <lance.lmwang@gmail.com>