backends/x11/windowed: Port to glReadnPixels()

Vlad Zahorodniirequested to merge
work/cherry-pick-1ffc72ef into Plasma/6.2

glReadnPixels() is a safer version of glReadPixels().

(cherry picked from commit 1ffc72ef)

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

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