LibWeb: Avoid dynamic_cast in DOM::Document::is_decoded_svg()

Libraries/LibWeb/DOM/Document.cpp

@@ -6033,7 +6033,7 @@ void Document::for_each_shadow_root(Function<void(DOM::ShadowRoot&)>&& callback)
 
 bool Document::is_decoded_svg() const
 {
-    return is<Web::SVG::SVGDecodedImageData::SVGPageClient>(page().client());
+    return page().client().is_svg_page_client();
 }
 
 // https://drafts.csswg.org/css-position-4/#add-an-element-to-the-top-layer

Libraries/LibWeb/Page/Page.h

@@ -408,6 +408,8 @@ public:
 
     virtual bool is_headless() const = 0;
 
+    virtual bool is_svg_page_client() const { return false; }
+
 protected:
     virtual ~PageClient() = default;
 };

Libraries/LibWeb/SVG/SVGDecodedImageData.h

@@ -92,6 +92,8 @@ private:
     {
     }
 
+    virtual bool is_svg_page_client() const override { return true; }
+
     virtual void visit_edges(Visitor&) override;
 };