lua : fix images reporting their height instead of their ID
lua : improve dumper

correctly detect more cases of duplicates, in particular images
lua/image : add group management

* _group_with(image,[image]) : puts the first image in the same group as the second image. If no second image is provided, the image will be in its own group
* _make_group_leader(image) : makes the image the leader of its group
* _get_group_members_ : returns a table containing all images of the group. The group leader is both at a numeric key and at the "leader" special key (so you probably want to use ipairs to iterate through that table)