decklink: Add 2.1 & 4.1 surround layouts

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The list of channel layouts available for decklink input is missing 2.1
& 4.1 layouts.  The commit adds them.  This aligns the decklink input
with the speaker layouts available at outputs.  Having different layouts
as input and output invokes FFmpeg resampler, which remixes the channels
in non trivial way except when downmixing to stereo.  This patch allows
to avoid such uncontrolled remix of channels with decklink input.

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