Playback speed not stored for scrolling or frame-based animations

[cibomahto]

Since we are using generic files for this, there isn't a place for it. PNG and JPG have sidecars for this but they get wiped during round trips through most photo editing software, so they won't work. A simple alternative would be filename based, but that is fragile.

Maybe time to make a patternpaint scene format? JSON+base64 image blobs might work.