On-demand resolution for thumbnails
Currently the thumbnails in lighttable mode are a tradeoff between size for generating them and quality needed for zoom/few thumbs per row.
One possibility to overcome this would be an on-demand creation of different mipmap levels. In general a coarse representation of the image could be generated/cached/used, but when needed a higher resolution is processed+cached and shown. Maybe that way we can have a fast and responsive lighttable which still allows to rate images using a high res preview.