Use viewport coordinate system

[iangilman]

(This is pretty much just a copy of altert/OpenseadragonFabricjsOverlay#18)

It appears that this plugin is using the image coordinate system for the first image in the collection. For multi-image, this can be problematic. I recommend standardizing on the viewport coordinate system instead. I suppose you could have a flag for backwards compatibility.

Also it's probably good to document which coordinate system the canvas overlay is using, regardless.