... _even at low zoom levels_

Pass GIMP_ZOOM_FOCUS_POINTER to gimp_display_shell_scale() when
wheel-scrolling, and change the scaling code to really honor
GIMP_ZOOM_FOCUS_POINTER and not apply magic image centering.

This keep the same point centered under the mouse for wheel-scrolling
and the zoom tool (== when the zooming is really triggered at a
certain mouse position).