scene: Add an assert to debug crash in BlurEffect::prePaintWindow()

Vlad Zahorodniirequested to merge
work/cherry-pick-b708a93c into Plasma/6.1

This checks whether a WindowItem is destroyed in the middle of a painting cycle.

This is meant as short-term means to debug the issue.

(cherry picked from commit b708a93c)

Co-authored-by: Vlad Zahorodnii vlad.zahorodnii@kde.org

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