LibWeb+WebContent: Move display list rasterization off the main thread

The display list is an immutable data structure, so once it's created,
rasterization can be moved to a separate thread. This allows more room
for performing other tasks between processing HTML rendering tasks.

This change makes PaintingSurface, ImmutableBitmap, and GlyphRun atomic
ref-counted, as they are shared between the main and rendering threads
by being included in the display list.

Libraries/LibWeb/HTML/RenderingThread.h

@@ -0,0 +1,52 @@
+/*
+ * Copyright (c) 2025, Aliaksandr Kalenik <kalenik.aliaksandr@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Noncopyable.h>
+#include <AK/Queue.h>
+#include <LibThreading/ConditionVariable.h>
+#include <LibThreading/Mutex.h>
+#include <LibThreading/Thread.h>
+#include <LibWeb/Painting/DisplayListPlayerSkia.h>
+
+namespace Web::HTML {
+
+class RenderingThread {
+    AK_MAKE_NONCOPYABLE(RenderingThread);
+    AK_MAKE_NONMOVABLE(RenderingThread);
+
+public:
+    RenderingThread();
+    ~RenderingThread();
+
+    void start();
+    void set_skia_player(OwnPtr<Painting::DisplayListPlayerSkia>&& player) { m_skia_player = move(player); }
+    void enqueue_rendering_task(RefPtr<Painting::DisplayList>, NonnullRefPtr<Gfx::PaintingSurface>, Function<void()>&& callback);
+
+private:
+    void rendering_thread_loop();
+
+    Core::EventLoop& m_main_thread_event_loop;
+
+    OwnPtr<Painting::DisplayListPlayerSkia> m_skia_player;
+
+    RefPtr<Threading::Thread> m_thread;
+    Atomic<bool> m_exit { false };
+
+    struct Task {
+        RefPtr<Painting::DisplayList> display_list;
+        NonnullRefPtr<Gfx::PaintingSurface> painting_surface;
+        Function<void()> callback;
+    };
+    // NOTE: Queue will only contain multiple items in case tasks were scheduled by screenshot requests.
+    //       Otherwise, it will contain only one item at a time.
+    Queue<Task> m_rendering_tasks;
+    Threading::Mutex m_rendering_task_mutex;
+    Threading::ConditionVariable m_rendering_task_ready_wake_condition { m_rendering_task_mutex };
+};
+
+}