magnifier: Use a ClutterContent to render mouse sprite

magnifier: Use a ClutterContent to render mouse sprite

The Magnifier class uses a small subtree of actors to track the
current cursor's position and sprite. Specifically, it uses the
deprecated ClutterTexture to paint the cursor sprites.

Add a new, very simple ClutterContent implementation to track the
cursor sprite, and replace the ClutterTexture by a ClutterActor.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/371