app: add GimpTileHandlerValidate::invalidated signal

Add a new GimpTileHandlerValidate::invalidated signal, which is
emitted when a region of the buffer is invalidated.  This would
allow us to properly invalidate the graph in response; this
normally happens in response to GeglBuffer::changed, but this
signal is not emitted when a region is merely invalidated.

(cherry picked from commit 3ea391602e)

app/gegl/gimptilehandlervalidate.c

@@ -22,11 +22,19 @@
 
 #include "gimp-gegl-types.h"
 
+#include "core/gimpmarshal.h"
+
 #include "gimp-gegl-loops.h"
 #include "gimp-gegl-utils.h"
 #include "gimptilehandlervalidate.h"
 
 
+enum
+{
+  INVALIDATED,
+  LAST_SIGNAL
+};
+
 enum
 {
   PROP_0,
@@ -71,12 +79,24 @@ G_DEFINE_TYPE (GimpTileHandlerValidate, gimp_tile_handler_validate,
 
 #define parent_class gimp_tile_handler_validate_parent_class
 
+static guint gimp_tile_handler_validate_signals[LAST_SIGNAL];
+
 
 static void
 gimp_tile_handler_validate_class_init (GimpTileHandlerValidateClass *klass)
 {
   GObjectClass *object_class = G_OBJECT_CLASS (klass);
 
+  gimp_tile_handler_validate_signals[INVALIDATED] =
+    g_signal_new ("invalidated",
+                  G_TYPE_FROM_CLASS (klass),
+                  G_SIGNAL_RUN_FIRST,
+                  G_STRUCT_OFFSET (GimpTileHandlerValidateClass, invalidated),
+                  NULL, NULL,
+                  g_cclosure_marshal_VOID__BOXED,
+                  G_TYPE_NONE, 1,
+                  GEGL_TYPE_RECTANGLE);
+
   object_class->finalize     = gimp_tile_handler_validate_finalize;
   object_class->set_property = gimp_tile_handler_validate_set_property;
   object_class->get_property = gimp_tile_handler_validate_get_property;
@@ -462,6 +482,9 @@ gimp_tile_handler_validate_invalidate (GimpTileHandlerValidate *validate,
                                 (cairo_rectangle_int_t *) rect);
 
   gegl_tile_handler_damage_rect (GEGL_TILE_HANDLER (validate), rect);
+
+  g_signal_emit (validate, gimp_tile_handler_validate_signals[INVALIDATED],
+                 0, rect, NULL);
 }
 
 void

app/gegl/gimptilehandlervalidate.h

@@ -56,6 +56,11 @@ struct _GimpTileHandlerValidateClass
 {
   GeglTileHandlerClass  parent_class;
 
+  /*  signals  */
+  void (* invalidated)     (GimpTileHandlerValidate *validate,
+                            const GeglRectangle     *rect);
+
+  /*  virtual functions  */
   void (* begin_validate)  (GimpTileHandlerValidate *validate);
   void (* end_validate)    (GimpTileHandlerValidate *validate);
   void (* validate)        (GimpTileHandlerValidate *validate,