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.
Place the rulers' origin at the top-left corner of the canvas
(screen space) bounding box, and set their scale to the image-
space scale along the screen-space horizontal/vertical directions
(in other words, measuring a distance using the rulers should
give the same results as the measure tool; note that rotation
comes into play here only when the horizontal and vertical
image or screen resolutions are different, since otherwise the
scale is direction invariant.)
Make scrollbar step match ruler step under the new behavior.
Ell