plugins/screencast: Fix crash in WindowScreenCastSource::render()

Vlad Zahorodniirequested to merge
work/zzag/screencast-crash into master

The transient list can contain popups that are not ready for painting yet and have no corresponding window items.

This change makes the WindowScreenCastSource add popups only in response to the Workspace::windowAdded signal, which roughly co-incides with the moment when the window is ready for painting.

SENTRY: KWIN-FZ3

Edited by Vlad Zahorodnii

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