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.cpp

@@ -95,7 +95,7 @@ void WebViewBridge::enqueue_input_event(Web::KeyEvent event)
 
 Optional<WebViewBridge::Paintable> WebViewBridge::paintable()
 {
-    Gfx::Bitmap* bitmap = nullptr;
+    Gfx::Bitmap const* bitmap = nullptr;
     Gfx::IntSize bitmap_size;
 
     if (m_client_state.has_usable_bitmap) {