wayland: fix pointer focus for destroyed surfaces

We had an assertion in meta_wayland_surface_free() that after
a repick() we would not choose the freed surface, but that didn't
consider surfaces destroyed while holding the implicit pointer
grab (ie, because the user clicked on the X button). In that case,
we need to bypass the grab infrastructure and explicitly unfocus
the dead surface.

https://bugzilla.gnome.org/show_bug.cgi?id=706982

src/wayland/meta-wayland-pointer.h

@@ -35,6 +35,10 @@ meta_wayland_pointer_set_focus (MetaWaylandPointer *pointer,
                                 MetaWaylandSurface *surface,
                                 wl_fixed_t sx,
                                 wl_fixed_t sy);
+
+void
+meta_wayland_pointer_destroy_focus (MetaWaylandPointer *pointer);
+
 void
 meta_wayland_pointer_start_grab (MetaWaylandPointer *pointer,
                                  MetaWaylandPointerGrab *grab);