lavf/mov.c: Fix timestamps to be strictly monotonic for video also.

We already do this for audio, but it should be done for video too.
If we don't, seeking back to the start of the file, for example, can
become quite broken, since the first N packets will have repeating
and nonmonotonic PTS, yet they need to be decoded even if they are
to be discarded.

Signed-off-by: Sasi Inguva <isasi@isasi.mtv.corp.google.com>
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>