wayland-pointer: fix condition for grabbing

It returns FALSE when button_count is not 0. But grabbing for
move/resize is activated by clicking the button, so this condition
disallows the wayland clients to be moved/resized.

https://bugzilla.gnome.org/show_bug.cgi?id=731237

src/wayland/meta-wayland-pointer.c

@@ -813,7 +813,7 @@ meta_wayland_pointer_can_grab_surface (MetaWaylandPointer *pointer,
                                        MetaWaylandSurface *surface,
                                        uint32_t            serial)
 {
-  return (pointer->button_count == 0 &&
+  return (pointer->button_count > 0 &&
           pointer->grab_serial == serial &&
           pointer->focus_surface == surface);
 }