wayland: Add MetaWaylandSurface::destroy signal

There's places where it would be convenient to add listeners to this,
so add the signal. The signal is only emitted once during destruction,
it is convenient for the places where we want notifications at a time
the object is still alive, as opposed to weak refs which notify after
the fact.

src/wayland/meta-wayland-surface.c

@@ -137,6 +137,13 @@ G_DEFINE_TYPE (MetaWaylandSurfaceRoleDND,
                meta_wayland_surface_role_dnd,
                META_TYPE_WAYLAND_SURFACE_ROLE);
 
+enum {
+  SURFACE_DESTROY,
+  N_SURFACE_SIGNALS
+};
+
+guint surface_signals[N_SURFACE_SIGNALS] = { 0 };
+
 static void
 meta_wayland_surface_role_assigned (MetaWaylandSurfaceRole *surface_role);
 
@@ -2670,6 +2677,20 @@ meta_wayland_surface_get_absolute_coordinates (MetaWaylandSurface *surface,
   *y = v.y;
 }
 
+static void
+meta_wayland_surface_dispose (GObject *object)
+{
+  MetaWaylandSurface *surface = META_WAYLAND_SURFACE (object);
+
+  if (!surface->destroying)
+    {
+      g_signal_emit (object, surface_signals[SURFACE_DESTROY], 0);
+      surface->destroying = TRUE;
+    }
+
+  G_OBJECT_CLASS (meta_wayland_surface_parent_class)->dispose (object);
+}
+
 static void
 meta_wayland_surface_init (MetaWaylandSurface *surface)
 {
@@ -2679,6 +2700,17 @@ meta_wayland_surface_init (MetaWaylandSurface *surface)
 static void
 meta_wayland_surface_class_init (MetaWaylandSurfaceClass *klass)
 {
+  GObjectClass *object_class = G_OBJECT_CLASS (klass);
+
+  object_class->dispose = meta_wayland_surface_dispose;
+
+  surface_signals[SURFACE_DESTROY] =
+    g_signal_new ("destroy",
+                  G_TYPE_FROM_CLASS (object_class),
+                  G_SIGNAL_RUN_LAST,
+                  0, NULL, NULL,
+                  g_cclosure_marshal_VOID__VOID,
+                  G_TYPE_NONE, 0);
 }
 
 static void

src/wayland/meta-wayland-surface.h

@@ -198,6 +198,7 @@ struct _MetaWaylandSurface
   MetaWaylandSerial acked_configure_serial;
   gboolean has_set_geometry;
   gboolean is_modal;
+  gboolean destroying;
 
   /* xdg_popup */
   struct {