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/LayoutBlock.h

@@ -41,7 +41,7 @@ public:
     virtual const char* class_name() const override { return "LayoutBlock"; }
 
     virtual void layout(LayoutMode = LayoutMode::Default) override;
-    virtual void render(RenderingContext&) override;
+    virtual void render(RenderingContext&, PaintPhase) override;
 
     virtual LayoutNode& inline_wrapper() override;