Make one buffersrc for each input and configure it to mono. Try to guess
the channel position from the port name or use the config option and
fall back to FC (MONO) if unspecified.
Make one buffersink for each output and place a format converter in
front of it. Configure the converter to produce 1 channel with a layout
guessed from the port name or from the config.
With this we can use channelsplit and amerge to create multichannel
streams for avfilter plugins.
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