LibGfx+LibWeb: Store Typeface and Font-related types in RefPtr to const

Author: https://github.com/ADKaster
Commit: be2dd91289
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4362
Reviewed-by: https://github.com/AtkinsSJ
Reviewed-by: https://github.com/Hendiadyoin1

Libraries/LibWeb/CSS/FontFace.h

@@ -107,8 +107,8 @@ private:
     Vector<ParsedFontFace::Source> m_urls;          // [[Urls]]
     ByteBuffer m_binary_data {};                    // [[Data]]
 
-    RefPtr<Gfx::Typeface> m_parsed_font;
-    RefPtr<Core::Promise<NonnullRefPtr<Gfx::Typeface>>> m_font_load_promise;
+    RefPtr<Gfx::Typeface const> m_parsed_font;
+    RefPtr<Core::Promise<NonnullRefPtr<Gfx::Typeface const>>> m_font_load_promise;
 
     // https://drafts.csswg.org/css-font-loading/#css-connected
     bool m_is_css_connected { false };