magnifier: Scale the sprite to match the current monitor scaling

magnifier: Scale the sprite to match the current monitor scaling

Compute the sprite texture scale (unfortunately using a workaround, based on
mutter cursor size preferences, tying to figure out the closest texture integer
scaling), and use this value to compute the scaling that should be applied to
the sprite in order to match the current monitor scaling.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/475>
(cherry picked from commit 484928e80acb4b6d161dcdd50d48e7731b4f9bee)