A few fixes and performance tweaks

[almarklein]

• 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.