This is mainly for the benefit of Mixbus, where input_latency
is not propagated upwards from the master bus (no direct
connection).
In Ardour's case _worst_input_latency >= _worst_route_latency
unless a given track with latent plugin is not connected.
Previously looping became out of sync (normal playback was not
affected) when a track had a latent plugin.
8a8ae7069e