output_all: synchronize playback with a notify object

Use audio_output_client_notify instead of g_usleep(1ms) in
audio_output_all_wait() to synchronize with the output_thread.  Signal
the audio_output_client_notify object in ao_play().

src/output_all.c

@@ -414,8 +414,7 @@ audio_output_all_wait(unsigned threshold)
 	if (audio_output_all_check() < threshold)
 		return true;
 
-	/* XXX synchronize in a better way */
-	g_usleep(1000);
+	notify_wait(&audio_output_client_notify);
 
 	return audio_output_all_check() < threshold;
 }

src/output_thread.c

@@ -155,6 +155,8 @@ static void ao_play(struct audio_output *ao)
 
 	ao->chunk_finished = true;
 	g_mutex_unlock(ao->mutex);
+
+	notify_signal(&audio_output_client_notify);
 }
 
 static void ao_pause(struct audio_output *ao)