screenshot: Flip captured images in place

Zhora Zmeykinrequested to merge
katze/kwin:screenshot-place-flip into master

Since !7997 (merged), the screenshot plugin renders the scene into a fresh offscreen target with a normal orientation. Therefore, all three call sites pass OutputTransform::Normal.

This small change uses QImage::flip(Qt::Vertical), which was introduced in Qt 6.9, instead of QImage::transformed().

This does not affect rotated outputs.

Performance

I compared both operations on a 4096x2160 QImage using QImage::Format_ARGB32_Premultiplied in a VM. After one warm-up iteration, the median times over 25 measured iterations were:

  • QImage::transformed(): 9.95 ms
  • QImage::flip(): 2.53 ms

In addition, QImage::flip() does not allocate a second image.

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