Due to a bug, the rate limiting did not happen for layout changes like zooming and panning.
Also made the rate limiting timeout larger for layout changes, because scroll zoom quickly feels jerky otherwise.
Fix the sometimes weird initialization by preventing initial calls for one callback.
When the image is larger in one dimension than in another, the thumbnails would be small in the smaller dimension. Changed so that the given size means the minimal size.
Also fixed a bug that the stored thumbnail size would sometimes be wrong.
Renamed lowres to thumbnail, because it can also contain fullres data.