Include two values for it, a default one that sets/keeps the current behavior,
where the frame event generated by the primary input will have a timestamp
equal or higher than frames in secondary input, plus a new one where the
secondary input frame will be that with the absolute closest timestamp to that
of the frame event one.

Addresses ticket #9689, where the new optional behavior produces better frame
syncronization.
Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: James Almer <jamrial@gmail.com>