Fix setting pretty-names when engine is restarted

port-meta-data depends on the audioengine backend/device
settings. Those are only available after the engine is started,
not from within the backend's _start() method.

This is is only relevant for callback based backends.

Backends with a blocking process thread explicitly emit
port-manager callbacks there before entering the main loop.

libs/backends/portaudio/portaudio_backend.cc

@@ -702,6 +702,7 @@ PortAudioBackend::_start (bool for_latency_measurement)
 			stop ();
 			return ProcessThreadStartError;
 		}
+		_port_change_flag.store (1);
 	}
 
 	return NoError;