Darkroom: when using shortcut to change zoom level, align image according to mouse pointer location
(Per conversation on FreeNode.)
<halley> if you hit "alt+1" with mouse on the image area, the 100% view could align with that area of the image. Otherwise it seems I always have to go pan to the area of interest.
This applies for any zoom level change done by a keyboard shortcut. If my mouse pointer is hovering over the subject's nose before zooming, then the zoom should be aligned so that the mouse is still hovering over the subject's nose after the change in scale. (Or as close as possible, if the image would be panned beyond its typical scrolling limits.) Currently, the zoom loupe is centered on the image.