Restored zoom box behavior

With the nice animated zoom change (#800), the ZoomBox was changed to use `zoomTo` instead of `zoomToExtent`.  The [calculation of the pixel origin](https://github.com/ahocevar/openlayers/commit/21448d2fd57d3d618a9cff4d1ded6a0e947eec5a#L2R105) (or anchor) for `zoomTo` was a bit off.  Fortunately, the tests caught this!  The change in behavior can also be seen in examples using the zoom box - after zooming, the map is not centered on the center of the drawn rectangle.

With the changes here, all tests pass again.  And the examples work as expected (confirmed with @bartvde's rare `out: true` setting as well :).