Implement Running Godot as Movie Writer

* Allows running the game in "movie writer" mode.
* It ensures entirely stable framerate, so your run can be saved stable and with proper sound (which is impossible if your CPU/GPU can't sustain doing this in real-time).
* If disabling vsync, it can save movies faster than the game is run, but if you want to control the interaction it can get difficult.
* Implements a simple, default MJPEG writer.

This new features has two main use cases, which have high demand:
* Saving game videos in high quality and ensuring the frame rate is *completely* stable, always.
* Using Godot as a tool to make movies and animations (which is ideal if you want interaction, or creating them procedurally. No other software is as good for this).

**Note**: This feature **IS NOT** for capturing real-time footage. Use something like OBS, SimpleScreenRecorder or FRAPS to achieve that, as they do a much better job at intercepting the compositor than Godot can probably do using Vulkan or OpenGL natively. If your game runs near real-time when capturing, you can still use this feature but it will play no sound (sound will be saved directly).

Usage:

$ godot --write-movie movie.avi [scene_file.tscn]

Missing:

* Options for configuring video writing via GLOBAL_DEF
* UI Menu for launching with this mode from the editor.
* Add to list of command line options.
* Add a feature tag to override configurations when movie writing (fantastic for saving videos with highest quality settings).

2 files changed, 75 insertions, 14 deletions

diff --git a/servers/audio/audio_driver_dummy.cpp b/servers/audio/audio_driver_dummy.cpp
index 635e4601c7..60eb657923 100644
--- a/servers/audio/audio_driver_dummy.cpp
+++ b/servers/audio/audio_driver_dummy.cpp
@@ -33,22 +33,24 @@
 #include "core/config/project_settings.h"
 #include "core/os/os.h"
 
+AudioDriverDummy *AudioDriverDummy::singleton = nullptr;
+
 Error AudioDriverDummy::init() {
 	active = false;
 	thread_exited = false;
 	exit_thread = false;
 	samples_in = nullptr;
 
-	mix_rate = GLOBAL_GET("audio/driver/mix_rate");
-	speaker_mode = SPEAKER_MODE_STEREO;
-	channels = 2;
-
-	int latency = GLOBAL_GET("audio/driver/output_latency");
-	buffer_frames = closest_power_of_2(latency * mix_rate / 1000);
+	if (mix_rate == -1) {
+		mix_rate = GLOBAL_GET("audio/driver/mix_rate");
+	}
 
+	channels = get_channels();
 	samples_in = memnew_arr(int32_t, (size_t)buffer_frames * channels);
 
-	thread.start(AudioDriverDummy::thread_func, this);
+	if (use_threads) {
+		thread.start(AudioDriverDummy::thread_func, this);
+	}
 
 	return OK;
 };
@@ -93,11 +95,56 @@ void AudioDriverDummy::unlock() {
 	mutex.unlock();
 };
 
+void AudioDriverDummy::set_use_threads(bool p_use_threads) {
+	use_threads = p_use_threads;
+}
+
+void AudioDriverDummy::set_speaker_mode(SpeakerMode p_mode) {
+	speaker_mode = p_mode;
+}
+
+void AudioDriverDummy::set_mix_rate(int p_rate) {
+	mix_rate = p_rate;
+}
+
+uint32_t AudioDriverDummy::get_channels() const {
+	static const int channels_for_mode[4] = { 2, 4, 8, 16 };
+	return channels_for_mode[speaker_mode];
+}
+
+void AudioDriverDummy::mix_audio(int p_frames, int32_t *p_buffer) {
+	ERR_FAIL_COND(!active); // If not active, should not mix.
+	ERR_FAIL_COND(use_threads == true); // If using threads, this will not work well.
+
+	uint32_t todo = p_frames;
+	while (todo) {
+		uint32_t to_mix = MIN(buffer_frames, todo);
+		lock();
+		audio_server_process(to_mix, samples_in);
+		unlock();
+
+		uint32_t total_samples = to_mix * channels;
+
+		for (uint32_t i = 0; i < total_samples; i++) {
+			p_buffer[i] = samples_in[i];
+		}
+
+		todo -= to_mix;
+		p_buffer += total_samples;
+	}
+}
+
 void AudioDriverDummy::finish() {
-	exit_thread = true;
-	thread.wait_to_finish();
+	if (use_threads) {
+		exit_thread = true;
+		thread.wait_to_finish();
+	}
 
 	if (samples_in) {
 		memdelete_arr(samples_in);
 	};
-};
+}
+
+AudioDriverDummy::AudioDriverDummy() {
+	singleton = this;
+}
diff --git a/servers/audio/audio_driver_dummy.h b/servers/audio/audio_driver_dummy.h
index 68b523e086..232a8d5e1f 100644
--- a/servers/audio/audio_driver_dummy.h
+++ b/servers/audio/audio_driver_dummy.h
@@ -44,9 +44,9 @@ class AudioDriverDummy : public AudioDriver {
 
 	static void thread_func(void *p_udata);
 
-	unsigned int buffer_frames;
-	unsigned int mix_rate;
-	SpeakerMode speaker_mode;
+	uint32_t buffer_frames = 4096;
+	int32_t mix_rate = -1;
+	SpeakerMode speaker_mode = SPEAKER_MODE_STEREO;
 
 	int channels;
 
@@ -54,6 +54,10 @@ class AudioDriverDummy : public AudioDriver {
 	bool thread_exited;
 	mutable bool exit_thread;
 
+	bool use_threads = true;
+
+	static AudioDriverDummy *singleton;
+
 public:
 	const char *get_name() const {
 		return "Dummy";
@@ -67,7 +71,17 @@ public:
 	virtual void unlock();
 	virtual void finish();
 
-	AudioDriverDummy() {}
+	void set_use_threads(bool p_use_threads);
+	void set_speaker_mode(SpeakerMode p_mode);
+	void set_mix_rate(int p_rate);
+
+	uint32_t get_channels() const;
+
+	void mix_audio(int p_frames, int32_t *p_buffer);
+
+	static AudioDriverDummy *get_dummy_singleton() { return singleton; }
+
+	AudioDriverDummy();
 	~AudioDriverDummy() {}
 };