Cropping: fix estimation of total file size of sub-stack
Cropping: fix estimation of total file size of sub-stack

The accumulation of the data read so far was reset for every slice. This was
wrong, because the estimation is checked against the limit for the whole file
(i.e. all slices).

Thanks to Felix von Zadow (@fzadow) for the pointer!

fzadow/CATMAID,fzadow/CATMAID,fzadow/CATMAID,htem/CATMAID,htem/CATMAID,fzadow/CATMAID,htem/CATMAID,htem/CATMAID