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/LayoutDocument.h

@@ -40,7 +40,9 @@ public:
     virtual const char* class_name() const override { return "LayoutDocument"; }
     virtual void layout(LayoutMode = LayoutMode::Default) override;
 
-    virtual void render(RenderingContext&) override;
+    void paint_all_phases(RenderingContext&);
+
+    virtual void render(RenderingContext&, PaintPhase) override;
 
     const LayoutRange& selection() const { return m_selection; }
     LayoutRange& selection() { return m_selection; }