windowitem: properly handle sub-subsurfaces
requested to merge work/zamundaaa/backport-43a9add5fec388c9f06363b624768dae3b669a02 into Plasma/5.27
The damage of the whole subsurface tree needs to be considered, not just the topmost subsurfaces of a window.
BUG: 466747 (cherry picked from commit 43a9add5)