magnifier: Update screen position on activation

magnifier: Update screen position on activation

The default ZoomRegion is created at startup and only updated if it is active
when the monitor setup changes. Thus when reactivating the magnifier after a
display change, the viewport used is still the one that 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.

Fixes https://gitlab.gnome.org/GNOME/gnome-shell/issues/1120