LibWeb: Add more fast_is<T> helpers for layout and paintable nodes

Author: https://github.com/awesomekling
Commit: 877391d569
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4395

Libraries/LibWeb/Layout/SVGGraphicsBox.h

@@ -23,6 +23,12 @@ public:
     SVG::SVGGraphicsElement const& dom_node() const { return as<SVG::SVGGraphicsElement>(SVGBox::dom_node()); }
 
     virtual GC::Ptr<Painting::Paintable> create_paintable() const override;
+
+private:
+    virtual bool is_svg_graphics_box() const override { return true; }
 };
 
+template<>
+inline bool Node::fast_is<SVGGraphicsBox>() const { return is_svg_graphics_box(); }
+
 }