magnifier: Avoid applying monitor scaling factors to the cursor twice

magnifier: Avoid applying monitor scaling factors to the cursor twice

The mouse actor needs special consideration because it is not a child
of the _uiGroupClone being magnified. So it gets magnified separately
*and* already has its own scaling factor from the source actor
(actually from the display scaling factor).

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/work_items/9302
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4290>
(cherry picked from commit b258fed0671d0f73a0308d42ab0af820dfab62cf)