fixes loading a session with tracks that have more input-ports
than output-ports.
the problem:
input -ports are added first, when added, IO::ensure_port() emits
IOChange::ConfigurationChanged which in turn triggers the route to
allocate corresponding output ports.
Due to this callback the output-ports were created before the actual
output-port-configuration and plugin-configuration from the
session-state was parsed and set.
5e1cfcc7ed