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

src/shell-util.h

@@ -44,9 +44,6 @@ GdkPixbuf *shell_util_create_pixbuf_from_data (const guchar      *data,
                                                int                height,
                                                int                rowstride);
 
-void    shell_util_cursor_tracker_to_clutter (MetaCursorTracker *tracker,
-                                              ClutterTexture    *texture);
-
 gboolean shell_util_need_background_refresh (void);
 
 ClutterContent * shell_util_get_content_for_window_actor (MetaWindowActor *window_actor,