avdevice/decklink: Add support for EIA-708 output over SDI

Hook in libklvanc and use it for output of EIA-708 captions over
SDI.  The bulk of this patch is just general support for ancillary
data for the Decklink SDI module - the real work for construction
of the EIA-708 CDP and VANC line construction is done by libklvanc.

Libklvanc can be found at: https://github.com/stoth68000/libklvanc



Updated to reflect feedback from Marton Balint <cus@passwd.hu>,
Carl Eugen Hoyos <ceffmpeg@gmail.com>, Aaron Levinson
<alevinsn_dev@levland.net>, and Moritz Barsnick <barsnick@gmx.net>

Signed-off-by: Devin Heitmueller <dheitmueller@ltnglobal.com>
Signed-off-by: Marton Balint <cus@passwd.hu>