tilelayer.js: fix excessive image loading upon resizing the canvas.
tilelayer.js: fix excessive image loading upon resizing the canvas.

Before, too many tiles loaded. One needed to zoom out and then zoom in
to trigger the code that corrected the URLs to a black gif when falling
completely off the field of view.

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