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/coreaudio/coreaudio_backend.cc

@@ -703,9 +703,8 @@ CoreAudioBackend::_start (bool for_latency_measurement)
 
 	engine.reconnect_ports ();
 
-	// force  an initial registration_callback() & latency re-compute
+	// force an initial registration_callback() & latency re-compute
 	_port_change_flag.store (1);
-	pre_process ();
 
 	_dsp_load_calc.reset ();
 	// all systems go.

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;