LibWeb: Separate layout tree rendering into phases

CSS defines a very specific paint order. This patch starts steering us
towards respecting that by introducing the PaintPhase enum with values:

- Background
- Border
- Foreground
- Overlay (internal overlays used by inspector)

Basically, to get the right visual result, we have to render the page
multiple times, going one phase at a time.

Libraries/LibWeb/Layout/LayoutBox.h

@@ -67,7 +67,7 @@ public:
     void set_stacking_context(NonnullOwnPtr<StackingContext> context) { m_stacking_context = move(context); }
     StackingContext* enclosing_stacking_context();
 
-    virtual void render(RenderingContext&) override;
+    virtual void render(RenderingContext&, PaintPhase) override;
 
 protected:
     LayoutBox(const Node* node, NonnullRefPtr<StyleProperties> style)