Previously Ardour only announced processor latency.
Routes that had additional latency to compensate for those
have not published this delay.
This is of no concern with internal backends, however with JACK,
Ardour reported incorrect *individual* port-latencies of
routes that perform PDC.
Since public port latency now includes delay-compensation,
some extra work is required to unset it before recalculating
latency of paths that include external ports.
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