Tile layer: fix same images on multiple stacks
Tile layer: fix same images on multiple stacks

Fix deferred tile loading (4481479a41d8e482147a78cd3719e1038454c770)
loading tile images from the last active stack in all opened stacks on
slice change. Thanks to @fzadow for the bug report.

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