decklink: Remove inactive audio channels (linux, macOs)

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Decklink cards accept 2, 8 or 16 audio channels.
If obs audio output is setup to n channels ( 8 >= n > 2), and the
decklink card captures n channels, one needs to squash the silent
channels (n+1, .., 8) or sampling issues occur.
This had been done on windows but is required also on macOs and linux.
The commit adds the fix for macOS and linux.
Some code factoring has also been done.

Closes obsproject/obs-studio#1350

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