Adds a driver for the BlackMagic DeckLink series of SDI cards (input only).
It requires BlackMagic's proprietary (but free-to-download) SDK to compile
and run. I've only tested it on Linux -- the Windows API is very similar but
not identical, so some adjustments will probably have to happen if anybody
wants this to work on Windows. (Windows users can, AFAIK, already access the
card via DirectShow, though.) It supports multiple cards, all the various
A/V inputs and video modes including setting field dominance and multichannel
input (2, 8 or 16 channels).

All testing has been with a DeckLink HD Extreme 3, which I have very
graciously been given access to by Frikanalen.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>