9b5f0cb8a2
New drag type `WM_DRAG_GREASE_PENCIL_GROUP` introduced for layer group nodes. Replaced layer struct with `GreasePencilLayerTreeNode` in `wmDragGreasePencilLayer` so group node can be stored while dragging in `wmdrag->poin`. Using `can_drop()`, drop operation of layer group node returns `false` if the target node is its child. Part of #121390. Pull Request: https://projects.blender.org/blender/blender/pulls/121654