Helper to debug re-render issues

It would help a lot to identify render problems if we could add a "Paint flashing" mode similar to Chrome's built-in tool.
[vokoscreenNG-2024-11-12_10-36-58.webm](https://github.com/user-attachments/assets/ca25261a-a01a-4572-a8cb-a147928c4bc6)
