Prevent "jumpiness" in zoomable lighttable view

This avoids unneeded position updates going from the recentcollect panel to the lighttable view. In zoomable mode these updates cause an ugly "jumpiness" of the thumbs display whenever the collection changes.

Additional fixes to make the string and buffer handling in collect/recentcollect more bullet proof.

I'd like someone to review these changes in order to not screw things shortly before the release.