cursor-tracker: Don't require a screen

This allows us to do initialization earlier and not have to poke into
Wayland internals from the cursor tracker.

src/backends/meta-cursor-tracker-private.h

@@ -30,7 +30,6 @@
 struct _MetaCursorTracker {
   GObject parent_instance;
 
-  MetaScreen *screen;
   MetaCursorRenderer *renderer;
 
   gboolean is_showing;