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/LayoutFrame.h

@@ -36,7 +36,7 @@ public:
     LayoutFrame(const Element&, NonnullRefPtr<StyleProperties>);
     virtual ~LayoutFrame() override;
 
-    virtual void render(RenderingContext&) override;
+    virtual void render(RenderingContext&, PaintPhase) override;
     virtual void layout(LayoutMode) override;
 
     const HTMLIFrameElement& node() const { return static_cast<const HTMLIFrameElement&>(LayoutReplaced::node()); }