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/Libraries/LibWeb/Painting/PaintableBox.h

@@ -194,7 +194,7 @@ public:
     void set_box_shadow_data(Vector<ShadowData> box_shadow_data) { m_box_shadow_data = move(box_shadow_data); }
     Vector<ShadowData> const& box_shadow_data() const { return m_box_shadow_data; }
 
-    PaintableBox const* nearest_scrollable_ancestor() const;
+    PaintableBox const* nearest_scrollable_ancestor_within_stacking_context() const;
 
 protected:
     explicit PaintableBox(Layout::Box const&);