69e520627f
Legacy Actions can become a layered Action, simply by removing all the legacy FCurves and adding a layer or binding. This means that `Action::idroot` can be non-zero, while it should be zero for layered Actions. This is now enforced in `Action::layer_add()` and `Action::binding_add()`. Pull Request: https://projects.blender.org/blender/blender/pulls/123004