fbc8504f9e swapped the iterators: Instead of iterating over output
buffers like BufferSet::read_from(), the new code iterates over
the current buffers. In case of 1 in -> 2 out, only the 1st channel
was be copied. Also unlike BufferSet::read_from() the ChanCount was
not updated.
Processors after a Delivery e.g. out-meter, AFL processor did not see
all channels.