acre-x: Fix zooming so that zoom-in and zoom-out use the same amounts

acre-x: Fix zooming so that zoom-in and zoom-out use the same amounts

The intent was always that one zoom-in followed by one zoom-out, (or
vice versa) would lead to the original zoom level. But the zoom-out
code had some incorrect arithmetic in it such that zooming out was
much faster than zooming in.