wayland: Don't unset surface->buffer when wl_buffer destroyed

Don't unset the surface->buffer if the associated wl_buffer object is
destroyed. The MetaWaylandBuffer doesn't really only represent a
wl_buffer object, but also the data (texture) created from the given
wl_buffer. Thus, for example destroying a released SHM wl_buffer should
not destroy the MetaWaylandBuffer instance, because the texture may
still be used.

This commit also fixes a race where calc_showing would hide a window
because, at the time of calculation whether it should be showing, the
surface's buffer had been destroyed as described above.

https://bugzilla.gnome.org/show_bug.cgi?id=762716

src/wayland/meta-wayland-surface.h

@@ -153,7 +153,6 @@ struct _MetaWaylandSurface
   MetaWindow *window;
   MetaWaylandBuffer *buffer;
   gboolean using_buffer;
-  struct wl_listener buffer_destroy_listener;
   cairo_region_t *input_region;
   cairo_region_t *opaque_region;
   int scale;