- store gm settings in the cache as a dict
- generalize usage of the gm settings to resize
- can be used to resize fullsize versions of the pictures
- without -quality switch, gm defaults to -quality 75.
use -define jpeg:preserve-settings for fullsize versions to
avoid altering them
- do not store "name" in the cache, because when the same image
is used in a gallery and as the cover, it invalidates the cache
entry and becomes always generated
