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/ViewportPaintable.cpp

@@ -57,4 +57,22 @@ void ViewportPaintable::paint_all_phases(PaintContext& context)
     stacking_context()->paint(context);
 }
 
+void ViewportPaintable::collect_scroll_frames(PaintContext& context) const
+{
+    i32 next_id = 0;
+    for_each_in_subtree_of_type<PaintableBox>([&](auto const& paintable_box) {
+        if (paintable_box.has_scrollable_overflow()) {
+            auto offset = paintable_box.scroll_offset();
+            auto ancestor = paintable_box.parent();
+            while (ancestor) {
+                if (ancestor->is_paintable_box() && static_cast<PaintableBox const*>(ancestor)->has_scrollable_overflow())
+                    offset.translate_by(static_cast<PaintableBox const*>(ancestor)->scroll_offset());
+                ancestor = ancestor->parent();
+            }
+            context.scroll_frames().set(&paintable_box, { .id = next_id++, .offset = -offset });
+        }
+        return TraversalDecision::Continue;
+    });
+}
+
 }