plugins/highlightwindow: Don't animate deleted or invisible windows

Sushi Trashrequested to merge
sssskeo/kwin:work/sssskeo/better-highlightwindow-performance into master

When there are many windows (>50) open across multiple virtual desktops HighlightWindowEffect becomes slow, causing annoying lag in prepareHighlighting and finishHighlighting.

There is no need to apply animations to deleted windows or those on a different desktop unless explicitly marked as highlighted.

Edited by Sushi Trash

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