Nodes: optimize freeing all nodes

We generally expect `bNodeTreeRuntime::nodes_by_id` to be valid at all times, so
it also has to have the same order `bNodeTree.nodes`. When freeing a node, the
entire vector set was rebuild currently to ensure that invariant. This leads
O(n^2) behavior when all nodes are freed as is commonly the case with depsgraph
copies etc.

This patch implements an optimization where `nodes_by_id` is not rebuild if only
the last node was removed. In this case, that not can just be popped from
`nodes_by_id` without affecting the order of the other nodes. To use this
optimization, the node tree freeing code now frees nodes in reverse order.

Pull Request: https://projects.blender.org/blender/blender/pulls/143831

source/blender/blenkernel/intern/node.cc

@@ -271,7 +271,9 @@ static void ntree_free_data(ID *id)
 
   BLI_freelistN(&ntree->links);
 
-  LISTBASE_FOREACH_MUTABLE (bNode *, node, &ntree->nodes) {
+  /* Iterate backwards because this allows for more efficient node deletion while keeping
+   * bNodeTreeRuntime::nodes_by_id valid. */
+  LISTBASE_FOREACH_BACKWARD_MUTABLE (bNode *, node, &ntree->nodes) {
     node_free_node(ntree, *node);
   }
 
@@ -4094,8 +4096,17 @@ void node_free_node(bNodeTree *ntree, bNode &node)
   /* can be called for nodes outside a node tree (e.g. clipboard) */
   if (ntree) {
     BLI_remlink(&ntree->nodes, &node);
-    /* Rebuild nodes #VectorSet which must have the same order as the list. */
-    node_rebuild_id_vector(*ntree);
+
+    const bool was_last = ntree->runtime->nodes_by_id.as_span().last() == &node;
+    if (was_last) {
+      /* No need to rebuild the entire bNodeTreeRuntime::nodes_by_id when the removed node is the
+       * last one. */
+      ntree->runtime->nodes_by_id.pop();
+    }
+    else {
+      /* Rebuild nodes #VectorSet which must have the same order as the list. */
+      node_rebuild_id_vector(*ntree);
+    }
 
     /* texture node has bad habit of keeping exec data around */
     if (ntree->type == NTREE_TEXTURE && ntree->runtime->execdata) {