Without a saveLayer, the BlendMode.plus will add itself to the backdrop and not just the previous image. Pushing without tests to see if existing goldens fail, but otherwise I have some good examples locally.

This is necessary uncondtionally, and lerping lerped images has the same issue.

Fixes https://github.com/flutter/flutter/issues/131617

### Before

![flutter_02](https://github.com/flutter/flutter/assets/8975114/1e783285-2fc2-429f-9fd8-6d04d4a155e1)

### After

![flutter_03](https://github.com/flutter/flutter/assets/8975114/3d08b187-26aa-4471-926d-e9ed5946a206)