some changes to expose more in our api

I am trying to migrate eigen's zoomable artwork image view to use ARTiledImageScrollView, which works well in Folio. I found myself wanting some more things available to me, so these are the changes I came up with. We can decide what does or doesn't make sense, this is just an idea. Here is the corresponding eigen PR, which is also a WIP https://github.com/artsy/eigen/pull/1971.