avdevice/decklink_dec: extract NTSC VANC

This changes how NTSC VANC is extracted from the buffer. In NTSC the vanc data
is interleaved between luma and chroma, and not just the luma as in high
definition resolutions.

In my testing this allows a decklink card encoding valid NTSC closed
captions to pass the caption data to the x264 encoder.

Updated with reviews from Devin Heitmueller and Marton Balint.

Signed-off-by: Ray Tiley <raytiley@gmail.com>
Signed-off-by: Marton Balint <cus@passwd.hu>