LibWeb: Coalesce nested repaint requests instead of swallowing them

Previously we would ignore repaint requests that came in via OOPWV while
the WebContent process was busy with a previous paint request.

This caused some easy-to-trigger bugs where the painted content would be
"one paint behind", especially noticeable when scrolling.

Userland/Libraries/LibWeb/OutOfProcessWebView.h

@@ -125,6 +125,7 @@ private:
         SharedBitmap back_bitmap;
         i32 next_bitmap_id { 0 };
         bool has_usable_bitmap { false };
+        bool got_repaint_requests_while_painting { false };
     } m_client_state;
 
     RefPtr<Gfx::Bitmap> m_backup_bitmap;