Add a button to reset position (move to center)

I find it a bit annoying that zooming out can put the image in some awkward corner.

Perhaps it's a good idea to prevent the image from going too far, e.g. ensure it's (1) always visible (so you can't scroll in a way which puts it off-screen) and (2) image edges snap to splitter if taken too much to the left or right.

So if the zoom or pan results in something like this:

...it would just snap to this: