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

Author: https://github.com/awesomekling
Commit: bbef0e8375
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4395

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;
 };