This fixes a bug where a world with multiple layers with different
"northdirection" values would initially load at the wrong center point
because the center for the last layer would override prior values
in the overview.collections.centers map.
I'd move the event setup into addTo, but JS is utter garbage and thinks
"this" in a function callback like that should be whatever, not where you
actually define it.
Instead of stuffing specialised values into overviewer.collections,
put a reference to the overviewerConfig tileset entry into the leaflet
layer, so that we can access it from callbacks easily.
Many things do not work at the moment:
* Missing world selector
* Missing overlays (overlay tilesets and markers)
* Missing urlhash code
* Probably other stuff
