app: add gimp_item_peek_node() and use it

so we can avoid peeking into the GimpItem struct if we want to get to
the item's node without implicitly creating it if it doesn't already
exist.

app/core/gimpdrawable.c

@@ -355,17 +355,20 @@ static void
 gimp_drawable_visibility_changed (GimpItem *item)
 {
   GimpDrawable *drawable = GIMP_DRAWABLE (item);
+  GeglNode     *item_node;
 
   /*  don't use gimp_item_get_node() because that would create
    *  the node.
    */
-  if (item->node)
+  item_node = gimp_item_peek_node (item);
+
+  if (item_node)
     {
       GeglNode *input;
       GeglNode *output;
 
-      input  = gegl_node_get_input_proxy  (item->node, "input");
+      input  = gegl_node_get_input_proxy  (item_node, "input");
-      output = gegl_node_get_output_proxy (item->node, "output");
+      output = gegl_node_get_output_proxy (item_node, "output");
 
       if (gimp_item_get_visible (item) &&
           ! (GIMP_IS_LAYER (item) &&

app/core/gimpitem.c

@@ -1421,6 +1421,14 @@ gimp_item_get_node (GimpItem *item)
   return GIMP_ITEM_GET_CLASS (item)->get_node (item);
 }
 
+GeglNode *
+gimp_item_peek_node (GimpItem *item)
+{
+  g_return_val_if_fail (GIMP_IS_ITEM (item), NULL);
+
+  return item->node;
+}
+
 GeglNode *
 gimp_item_get_offset_node (GimpItem *item)
 {

app/core/gimpitem.h

@@ -247,6 +247,7 @@ gboolean        gimp_item_stroke             (GimpItem           *item,
                                               GError            **error);
 
 GeglNode      * gimp_item_get_node           (GimpItem           *item);
+GeglNode      * gimp_item_peek_node          (GimpItem           *item);
 GeglNode      * gimp_item_get_offset_node    (GimpItem           *item);