Vlad Danilov

Some images show visible gradient banding / blocking artifacts even after cb5e4a8. Here's the crop to illustrate the issue.  From left to right: [Guetzli q94](https://user-images.githubusercontent.com/3129436/48576440-9ead0100-e925-11e8-84f1-d78d4460056d.jpg), [Original](https://user-images.githubusercontent.com/3129436/48575798-f21e4f80-e923-11e8-9ded-aa9612ae882c.jpg). This issue is...