LibWeb: Give InlineNodes a background

This now uses the same code as Box, so images, repeat-rules, and
anything that's added in the future will work for `display: inline`
elements too. :^)

Userland/Libraries/LibWeb/Layout/InlineNode.h

@@ -18,6 +18,10 @@ public:
     virtual void paint(PaintContext&, PaintPhase) override;
 
     virtual void split_into_lines(InlineFormattingContext&, LayoutMode) override;
+
+private:
+    template<typename Callback>
+    void for_each_fragment(Callback);
 };
 
 }