Bug fix: mouse-wheel zoom no longer gets stuck

Bug fix. Mouse-wheel zoom could end up in a strange state: after zooming in off to one side and back out, the map could stay off-center, and the next zoom-in would jump somewhere unexpected. Zoom now always keeps the spot under your cursor in place as you zoom in, returns to the exact full-map view when you zoom fully out, and no longer pans past the edges of the map. It also scales with how far you scroll, so a trackpad zooms smoothly instead of in big steps.