MetaWaylandPointerConstraints: Require pointer focus to enable

Require pointer focus to enable, otherwise we can't guarantee it has
entered the surface, as the focus may have been given to a subsurface,
override-redirect or other sub window covering the surface that was
requested to have o pointer constraint.

https://bugzilla.gnome.org/show_bug.cgi?id=771050

src/wayland/meta-wayland-pointer-constraints.c

@@ -436,6 +436,9 @@ meta_wayland_pointer_constraint_maybe_enable (MetaWaylandPointerConstraint *cons
   if (constraint->is_enabled)
     return;
 
+  if (constraint->seat->pointer->focus_surface != constraint->surface)
+    return;
+
   if (!constraint->surface->window)
     {
       /*