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/xaudio2/audio_driver_xaudio2.cpp

@@ -45,7 +45,7 @@ Error AudioDriverXAudio2::init() {
 	speaker_mode = SPEAKER_MODE_STEREO;
 	channels = 2;
 
-	int latency = GLOBAL_GET("audio/driver/output_latency");
+	int latency = Engine::get_singleton()->get_audio_output_latency();
 	buffer_size = closest_power_of_2(latency * mix_rate / 1000);
 
 	samples_in = memnew_arr(int32_t, buffer_size * channels);