The images used by Gitea's drone pipeline were upgraded to Go 1.19.x
It causes the lint fails because Go 1.19 uses new code format.
This PR partially backport #20758 (including the emoji-data sync),
partially fix the format manually.
This PR fixes the builtin avatar generator.
1. The random background color makes some images very dirty. So now we only use white background for avatars.
2. We use left-right mirror avatars to satisfy #14799
3. Fix a small padding error in the algorithm