LibGfx+LibWebView+UI: Store Gfx::Bitmap in RefPtr to const

Author: https://github.com/ADKaster
Commit: 91b549f797
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4362
Reviewed-by: https://github.com/AtkinsSJ
Reviewed-by: https://github.com/Hendiadyoin1

UI/AppKit/Interface/LadybirdWebViewBridge.h

@@ -42,7 +42,7 @@ public:
     void enqueue_input_event(Web::KeyEvent);
 
     struct Paintable {
-        Gfx::Bitmap& bitmap;
+        Gfx::Bitmap const& bitmap;
         Gfx::IntSize bitmap_size;
     };
     Optional<Paintable> paintable();