Make Servers truly Thread Safe

-Rendering server now uses a split RID allocate/initialize internally, this allows generating RIDs immediately but initialization to happen later on the proper thread (as rendering APIs generally requiere to call on the right thread).
-RenderingServerWrapMT is no more, multithreading is done in RenderingServerDefault.
-Some functions like texture or mesh creation, when renderer supports it, can register and return immediately (so no waiting for server API to flush, and saving staging and command buffer memory).
-3D physics server changed to be made multithread friendly.
-Added PhysicsServer3DWrapMT to use 3D physics server from multiple threads.
-Disablet Bullet (too much effort to make multithread friendly, this needs to be fixed eventually).

servers/rendering/rendering_server_default.cpp

@@ -64,7 +64,7 @@ void RenderingServerDefault::_draw_margins() {
 
 /* FREE */
 
-void RenderingServerDefault::free(RID p_rid) {
+void RenderingServerDefault::_free(RID p_rid) {
 	if (RSG::storage->free(p_rid)) {
 		return;
 	}
@@ -91,7 +91,7 @@ void RenderingServerDefault::request_frame_drawn_callback(Object *p_where, const
 	frame_drawn_callbacks.push_back(fdc);
 }
 
-void RenderingServerDefault::draw(bool p_swap_buffers, double frame_step) {
+void RenderingServerDefault::_draw(bool p_swap_buffers, double frame_step) {
 	//needs to be done before changes is reset to 0, to not force the editor to redraw
 	RS::get_singleton()->emit_signal("frame_pre_draw");
 
@@ -213,18 +213,15 @@ float RenderingServerDefault::get_frame_setup_time_cpu() const {
 	return frame_setup_time;
 }
 
-void RenderingServerDefault::sync() {
-}
-
 bool RenderingServerDefault::has_changed() const {
 	return changes > 0;
 }
 
-void RenderingServerDefault::init() {
+void RenderingServerDefault::_init() {
 	RSG::rasterizer->initialize();
 }
 
-void RenderingServerDefault::finish() {
+void RenderingServerDefault::_finish() {
 	if (test_cube.is_valid()) {
 		free(test_cube);
 	}
@@ -232,6 +229,32 @@ void RenderingServerDefault::finish() {
 	RSG::rasterizer->finalize();
 }
 
+void RenderingServerDefault::init() {
+	if (create_thread) {
+		print_verbose("RenderingServerWrapMT: Creating render thread");
+		DisplayServer::get_singleton()->release_rendering_thread();
+		if (create_thread) {
+			thread.start(_thread_callback, this);
+			print_verbose("RenderingServerWrapMT: Starting render thread");
+		}
+		while (!draw_thread_up) {
+			OS::get_singleton()->delay_usec(1000);
+		}
+		print_verbose("RenderingServerWrapMT: Finished render thread");
+	} else {
+		_init();
+	}
+}
+
+void RenderingServerDefault::finish() {
+	if (create_thread) {
+		command_queue.push(this, &RenderingServerDefault::_thread_exit);
+		thread.wait_to_finish();
+	} else {
+		_finish();
+	}
+}
+
 /* STATUS INFORMATION */
 
 int RenderingServerDefault::get_render_info(RenderInfo p_info) {
@@ -297,10 +320,6 @@ void RenderingServerDefault::set_debug_generate_wireframes(bool p_generate) {
 	RSG::storage->set_debug_generate_wireframes(p_generate);
 }
 
-void RenderingServerDefault::call_set_use_vsync(bool p_enable) {
-	DisplayServer::get_singleton()->_set_use_vsync(p_enable);
-}
-
 bool RenderingServerDefault::is_low_end() const {
 	// FIXME: Commented out when rebasing vulkan branch on master,
 	// causes a crash, it seems rasterizer is not initialized yet the
@@ -309,7 +328,77 @@ bool RenderingServerDefault::is_low_end() const {
 	return false;
 }
 
-RenderingServerDefault::RenderingServerDefault() {
+void RenderingServerDefault::_thread_exit() {
+	exit = true;
+}
+
+void RenderingServerDefault::_thread_draw(bool p_swap_buffers, double frame_step) {
+	if (!atomic_decrement(&draw_pending)) {
+		_draw(p_swap_buffers, frame_step);
+	}
+}
+
+void RenderingServerDefault::_thread_flush() {
+	atomic_decrement(&draw_pending);
+}
+
+void RenderingServerDefault::_thread_callback(void *_instance) {
+	RenderingServerDefault *vsmt = reinterpret_cast<RenderingServerDefault *>(_instance);
+
+	vsmt->_thread_loop();
+}
+
+void RenderingServerDefault::_thread_loop() {
+	server_thread = Thread::get_caller_id();
+
+	DisplayServer::get_singleton()->make_rendering_thread();
+
+	_init();
+
+	exit = false;
+	draw_thread_up = true;
+	while (!exit) {
+		// flush commands one by one, until exit is requested
+		command_queue.wait_and_flush_one();
+	}
+
+	command_queue.flush_all(); // flush all
+
+	_finish();
+}
+
+/* EVENT QUEUING */
+
+void RenderingServerDefault::sync() {
+	if (create_thread) {
+		atomic_increment(&draw_pending);
+		command_queue.push_and_sync(this, &RenderingServerDefault::_thread_flush);
+	} else {
+		command_queue.flush_all(); //flush all pending from other threads
+	}
+}
+
+void RenderingServerDefault::draw(bool p_swap_buffers, double frame_step) {
+	if (create_thread) {
+		atomic_increment(&draw_pending);
+		command_queue.push(this, &RenderingServerDefault::_thread_draw, p_swap_buffers, frame_step);
+	} else {
+		_draw(p_swap_buffers, frame_step);
+	}
+}
+
+RenderingServerDefault::RenderingServerDefault(bool p_create_thread) :
+		command_queue(p_create_thread) {
+	create_thread = p_create_thread;
+	draw_pending = 0;
+	draw_thread_up = false;
+
+	if (!p_create_thread) {
+		server_thread = Thread::get_caller_id();
+	} else {
+		server_thread = 0;
+	}
+
 	RSG::canvas = memnew(RendererCanvasCull);
 	RSG::viewport = memnew(RendererViewport);
 	RendererSceneCull *sr = memnew(RendererSceneCull);