The previous approach failed in case where PluginInsert
uses no-inplace buffers with a linear map.
Since buffers are replicated up to a total of number of
all (inputs + outputs), the number of output buffers
could not be determined. There was insufficient information
using the I/O map alone.
With a known number of outputs processing and applying
the i/o map is also a lot easier and faster.
This break the API of process_map().
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