tilelayer: tileSourceType 4 can also show the small overview.
tilelayer: tileSourceType 4 can also show the small overview.

The tileSourceType should be refactored into its own object with methods
to create the tile URL and the small overview URL.

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