backends: Dissociate visibility from current cursor sprite

Just like X11/XFixes behaves, the current cursor is not affected
by its visibility, so it can be queried while invisible (possibly
to be replaced).

For this, keep an extra effective_cursor pointer that will be
either equal to displayed_cursor (maybe a bit of a misnomer now)
or NULL if the cursor is invisible. The MetaCursorRenderer
management is tied to the former, and the ::cursor-changed signal
emission to the latter.

https://bugzilla.gnome.org/show_bug.cgi?id=754806

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

@@ -32,6 +32,7 @@ struct _MetaCursorTracker {
 
   gboolean is_showing;
 
+  MetaCursorSprite *effective_cursor; /* May be NULL when hidden */
   MetaCursorSprite *displayed_cursor;
 
   /* Wayland clients can set a NULL buffer as their cursor