In "show all" mode, the image is thought to be "infinite"; this
commit improves the overall scrolling/zooming behavior in this mode
according to this assumption.  In cases where a specific image size
is needed (e.g., for the scrollbar bounds, fit-image-in-window,
etc.), the image's full bounding box is used; however, in cases
where a center point is needed (e.g., for the zoomed-out scrollbar
bounds, center-image-in-window), the canvas center, rather than the
bounding-box center, is still used.