LibWeb: Let paintables cache their containing block and absolute rect

The absolute rect of a paintable is somewhat expensive to compute. This
is because all coordinates are relative to the nearest containing block,
so we have to traverse the containing block chain and apply each offset
to get the final rect.

Paintables will never move between containing blocks, nor will their
absolute rect change. If anything changes, we'll simpl make a new
paintable and replace the old one.

Take advantage of this by caching the containing block and absolute rect
after first access.

Userland/Libraries/LibWeb/Painting/PaintableBox.h

@@ -131,6 +131,8 @@ private:
     Painting::BorderRadiusData normalized_border_radius_data() const;
 
     OwnPtr<Painting::StackingContext> m_stacking_context;
+
+    Optional<Gfx::FloatRect> mutable m_absolute_rect;
 };
 
 class PaintableWithLines : public PaintableBox {