Fix T94162: incorrect handling when there are multiple group outputs

Typically a node group should only have a single Group Output node.
However, currently Blender already supports having multiple group outputs,
one of which is active. This wasn't handled correctly by geometry nodes.

Differential Revision: https://developer.blender.org/D13611

source/blender/nodes/NOD_node_tree_ref.hh

@@ -262,6 +262,7 @@ class NodeTreeRef : NonCopyable, NonMovable {
   Vector<LinkRef *> links_;
   MultiValueMap<const bNodeType *, NodeRef *> nodes_by_type_;
   Vector<std::unique_ptr<SocketIndexByIdentifierMap>> owned_identifier_maps_;
+  const NodeRef *group_output_node_ = nullptr;
 
  public:
   NodeTreeRef(bNodeTree *btree);
@@ -279,6 +280,11 @@ class NodeTreeRef : NonCopyable, NonMovable {
 
   const NodeRef *find_node(const bNode &bnode) const;
 
+  /**
+   * This is the active group output node if there are multiple.
+   */
+  const NodeRef *group_output_node() const;
+
   /**
    * \return True when there is a link cycle. Unavailable sockets are ignored.
    */
@@ -759,6 +765,11 @@ inline Span<const LinkRef *> NodeTreeRef::links() const
   return links_;
 }
 
+inline const NodeRef *NodeTreeRef::group_output_node() const
+{
+  return group_output_node_;
+}
+
 inline bNodeTree *NodeTreeRef::btree() const
 {
   return btree_;