Centered nodes in groups aren't relative to the group center

When having nodes with `LEFT`, `RIGHT`, `LEFT_RIGHT`, `BOTTOM`, `TOP`, `TOP_BOTTOM` contraints, the offset is relative to the first parent that is not a `GROUP`.

But when having a `CENTER` constraint, the offset must be relative to the center of the `GROUP`.

Currently the offset is also relative to the first parent that is not a `GROUP`.