Add a --audio-output-latency command-line argument

This allows optimizing the audio output latency on higher-end CPUs,
especially in projects that do not expose a way to override this setting.

drivers/pulseaudio/audio_driver_pulseaudio.cpp

@@ -222,7 +222,7 @@ Error AudioDriverPulseAudio::init_output_device() {
 			break;
 	}
 
-	int tmp_latency = GLOBAL_GET("audio/driver/output_latency");
+	int tmp_latency = Engine::get_singleton()->get_audio_output_latency();
 	buffer_frames = closest_power_of_2(tmp_latency * mix_rate / 1000);
 	pa_buffer_size = buffer_frames * pa_map.channels;