Improved touch support for Handler.Path.undo

In OpenLayers.Handler.Path.undo on touch devices, set the current ("mouse location") point to match the last digitized point.  Touch devices have no current mouse position (just a last-tapped position), and it's misleading to leave the current point sitting at the last-tapped position.

There's currently no built-in UI for invoking undo on touch devices, but for developers (like me) who are calling into the handler from their code, this fix is helpful.