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.cpp

@@ -0,0 +1,67 @@
+/*
+ * Copyright (c) 2025, Aliaksandr Kalenik <kalenik.aliaksandr@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibCore/EventLoop.h>
+#include <LibWeb/HTML/RenderingThread.h>
+
+namespace Web::HTML {
+
+RenderingThread::RenderingThread()
+    : m_main_thread_event_loop(Core::EventLoop::current())
+{
+}
+
+RenderingThread::~RenderingThread()
+{
+    m_exit = true;
+    m_rendering_task_ready_wake_condition.signal();
+    (void)m_thread->join();
+}
+
+void RenderingThread::start()
+{
+    VERIFY(m_skia_player);
+    m_thread = Threading::Thread::construct([this] {
+        rendering_thread_loop();
+        return static_cast<intptr_t>(0);
+    });
+    m_thread->start();
+}
+
+void RenderingThread::rendering_thread_loop()
+{
+    while (true) {
+        auto task = [this]() -> Optional<Task> {
+            Threading::MutexLocker const locker { m_rendering_task_mutex };
+            while (m_rendering_tasks.is_empty() && !m_exit) {
+                m_rendering_task_ready_wake_condition.wait();
+            }
+            if (m_exit)
+                return {};
+            return m_rendering_tasks.dequeue();
+        }();
+
+        if (!task.has_value()) {
+            VERIFY(m_exit);
+            break;
+        }
+
+        m_skia_player->set_surface(task->painting_surface);
+        m_skia_player->execute(*task->display_list);
+        m_main_thread_event_loop.deferred_invoke([callback = move(task->callback)] {
+            callback();
+        });
+    }
+}
+
+void RenderingThread::enqueue_rendering_task(RefPtr<Painting::DisplayList> display_list, NonnullRefPtr<Gfx::PaintingSurface> painting_surface, Function<void()>&& callback)
+{
+    Threading::MutexLocker const locker { m_rendering_task_mutex };
+    m_rendering_tasks.enqueue(Task { move(display_list), move(painting_surface), move(callback) });
+    m_rendering_task_ready_wake_condition.signal();
+}
+
+}