Add support for more channel combinations.
Add support for more channel combinations.

Decklink can only output 2, 8 or 16 channels. This change will drop
channels or add silent channels to match one of the supported channel
numbers.

mltframework/mlt,mltframework/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,j-b-m/mlt,j-b-m/mlt,j-b-m/mlt,j-b-m/mlt,mltframework/mlt,mltframework/mlt,mltframework/mlt,j-b-m/mlt,j-b-m/mlt,j-b-m/mlt,j-b-m/mlt,mltframework/mlt,mltframework/mlt,mltframework/mlt,j-b-m/mlt