LibWeb: Make OOPWV resizing flicker-free :^)

When resizing, keep a copy of the last good bitmap we have, and keep
using that until we receive a new one from the WebContent process.

Userland/Libraries/LibWeb/OutOfProcessWebView.h

@@ -99,6 +99,8 @@ private:
         i32 next_bitmap_id { 0 };
         bool has_usable_bitmap { false };
     } m_client_state;
+
+    RefPtr<Gfx::Bitmap> m_backup_bitmap;
 };
 
 }