Forward data for ports that are both internally and
externally connected.
e.g. click-io may be connected to a track as well as physical
outputs. Since the port is externally connected, data is written
to an internal buffer _data[], and only at cycle-end resampled
to engine-buffers.
However a track's recording the metronome is not externally
connected. Hence data is directly read from the engine-buffers.
Summing also happens at engine level, so data has to
be written back for downstream ports to read them in the same cycle.
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