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/LayoutCanvas.h

@@ -39,7 +39,7 @@ public:
     virtual ~LayoutCanvas() override;
 
     virtual void layout(LayoutMode = LayoutMode::Default) override;
-    virtual void render(RenderingContext&) override;
+    virtual void render(RenderingContext&, PaintPhase) override;
 
     const HTMLCanvasElement& node() const { return static_cast<const HTMLCanvasElement&>(LayoutReplaced::node()); }