Add utility function to copy default values from one tree to another

The trees should match in nodes layout completely.

source/blender/blenkernel/BKE_node.h

@@ -1050,4 +1050,11 @@ int ntreeTexExecTree(struct bNodeTree *ntree, struct TexResult *target,
 void init_nodesystem(void);
 void free_nodesystem(void);
 
+/* -------------------------------------------------------------------- */
+/* evaluation support, */
+
+void BKE_nodetree_copy_default_values(struct bNodeTree *ntree_dst,
+                                      const struct bNodeTree *ntree_src);
+
+
 #endif  /* __BKE_NODE_H__ */

source/blender/blenkernel/intern/node.c

@@ -3804,3 +3804,33 @@ void BKE_nodetree_remove_layer_n(bNodeTree *ntree, Scene *scene, const int layer
 		}
 	}
 }
+
+static void node_copy_default_values_list(ListBase *sockets_dst,
+                                          const ListBase *sockets_src)
+{
+	bNodeSocket *sock_dst = sockets_dst->first;
+	const bNodeSocket *sock_src = sockets_src->first;
+	while (sock_dst != NULL) {
+		node_socket_copy_default_value(sock_dst, sock_src);
+		sock_dst = sock_dst->next;
+		sock_src = sock_src->next;
+	}
+}
+
+static void node_copy_default_values(bNode *node_dst, const bNode *node_src)
+{
+	node_copy_default_values_list(&node_dst->inputs, &node_src->inputs);
+	node_copy_default_values_list(&node_dst->outputs, &node_src->outputs);
+}
+
+void BKE_nodetree_copy_default_values(bNodeTree *ntree_dst,
+                                      const bNodeTree *ntree_src)
+{
+	bNode *node_dst = ntree_dst->nodes.first;
+	const bNode *node_src = ntree_src->nodes.first;
+	while (node_dst != NULL) {
+		node_copy_default_values(node_dst, node_src);
+		node_dst = node_dst->next;
+		node_src = node_src->next;
+	}
+}

source/blender/nodes/NOD_socket.h

@@ -49,7 +49,7 @@ struct bNodeSocket *node_add_socket_from_template(struct bNodeTree *ntree, struc
 void node_verify_socket_templates(struct bNodeTree *ntree, struct bNode *node);
 
 void node_socket_init_default_value(struct bNodeSocket *sock);
-void node_socket_copy_default_value(struct bNodeSocket *to, struct bNodeSocket *from);
+void node_socket_copy_default_value(struct bNodeSocket *to, const struct bNodeSocket *from);
 void register_standard_node_socket_types(void);
 
 #endif

source/blender/nodes/intern/node_socket.c

@@ -267,7 +267,7 @@ void node_socket_init_default_value(bNodeSocket *sock)
 	}
 }
 
-void node_socket_copy_default_value(bNodeSocket *to, bNodeSocket *from)
+void node_socket_copy_default_value(bNodeSocket *to, const bNodeSocket *from)
 {
 	/* sanity check */
 	if (to->type != from->type)