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).

servers/rendering/renderer_viewport.cpp

@@ -1193,6 +1193,20 @@ void RendererViewport::viewport_set_sdf_oversize_and_scale(RID p_viewport, RS::V
 	RSG::texture_storage->render_target_set_sdf_size_and_scale(viewport->render_target, p_size, p_scale);
 }
 
+RID RendererViewport::viewport_find_from_screen_attachment(DisplayServer::WindowID p_id) const {
+	RID *rids = nullptr;
+	uint32_t rid_count = viewport_owner.get_rid_count();
+	rids = (RID *)alloca(sizeof(RID *) * rid_count);
+	viewport_owner.fill_owned_buffer(rids);
+	for (uint32_t i = 0; i < rid_count; i++) {
+		Viewport *viewport = viewport_owner.get_or_null(rids[i]);
+		if (viewport->viewport_to_screen == p_id) {
+			return rids[i];
+		}
+	}
+	return RID();
+}
+
 bool RendererViewport::free(RID p_rid) {
 	if (viewport_owner.owns(p_rid)) {
 		Viewport *viewport = viewport_owner.get_or_null(p_rid);

servers/rendering/renderer_viewport.h

@@ -282,6 +282,8 @@ public:
 
 	void viewport_set_sdf_oversize_and_scale(RID p_viewport, RS::ViewportSDFOversize p_over_size, RS::ViewportSDFScale p_scale);
 
+	virtual RID viewport_find_from_screen_attachment(DisplayServer::WindowID p_id = DisplayServer::MAIN_WINDOW_ID) const;
+
 	void handle_timestamp(String p_timestamp, uint64_t p_cpu_time, uint64_t p_gpu_time);
 
 	void set_default_clear_color(const Color &p_color);

servers/rendering/rendering_server_default.h

@@ -629,6 +629,7 @@ public:
 	FUNC2(viewport_set_measure_render_time, RID, bool)
 	FUNC1RC(double, viewport_get_measured_render_time_cpu, RID)
 	FUNC1RC(double, viewport_get_measured_render_time_gpu, RID)
+	FUNC1RC(RID, viewport_find_from_screen_attachment, DisplayServer::WindowID)
 
 	FUNC2(call_set_vsync_mode, DisplayServer::VSyncMode, DisplayServer::WindowID)