plugins/screencast: close screencasts on GPU reset

Xaver Huglrequested to merge
work/zamundaaa/gpu-reset-screencast into master

Ideally we'd make the scene persistent and handle GPU resets in the screencast plugin, but until that's done, closing the screencast is better than crashing.

Fixes https://crash-reports.kde.org/share/issue/73d294e21e53493798c996509bdc5240/

BUG: 513710

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