magnifier: Update screen position on activation

The default ZoomRegion is created at startup and not updated if the monitor setup changes unless is active. Thus when reactivating the magnifier after a display change, the viewport has still been computed with the old screen geometry values.

Move screen update code inside a function and call it both when activating the zoom region and when the monitor changes during a zoom session.

Finally connect to signals only when the Zoom region is active.