LibWeb: Apply scroll boxes offsets after painting commands recording

With this change, instead of applying scroll offsets during the
recording of the painting command list, we do the following:
1. Collect all boxes with scrollable overflow into a PaintContext,
   each with an id and the total amount of scrolling offset accumulated
   from ancestor scrollable boxes.
2. During the recording phase assign a corresponding scroll_frame_id to
   each command that paints content within a scrollable box.
3. Before executing the recorded commands, translate each command that
   has a scroll_frame_id by the accumulated scroll offset.

This approach has following advantages:
- Implementing nested scrollables becomes much simpler, as the
  recording phase only requires the correct assignment of the nearest
  scrollable's scroll_frame_id, while the accumulated offset from
  ancestors is applied subsequently.
- The recording of painting commands is not tied to a specific offset
  within scrollable boxes, which means in the future, it will be
  possible to update the scrolling offset and repaint without the need
  to re-record painting commands.

Userland/Services/WebContent/PageClient.cpp

@@ -188,8 +188,16 @@ void PageClient::paint(Web::DevicePixelRect const& content_rect, Gfx::Bitmap& ta
     context.set_should_paint_overlay(paint_options.paint_overlay == Web::PaintOptions::PaintOverlay::Yes);
     context.set_device_viewport_rect(content_rect);
     context.set_has_focus(m_has_focus);
+
+    document->paintable()->collect_scroll_frames(context);
     document->paintable()->paint_all_phases(context);
 
+    Vector<Gfx::IntPoint> scroll_offsets_by_frame_id;
+    scroll_offsets_by_frame_id.resize(context.scroll_frames().size());
+    for (auto [_, scrollable_frame] : context.scroll_frames())
+        scroll_offsets_by_frame_id[scrollable_frame.id] = context.rounded_device_point(scrollable_frame.offset).to_type<int>();
+    recording_painter.apply_scroll_offsets(scroll_offsets_by_frame_id);
+
     if (s_use_gpu_painter) {
 #ifdef HAS_ACCELERATED_GRAPHICS
         Web::Painting::PaintingCommandExecutorGPU painting_command_executor(*m_accelerated_graphics_context, target);