Make sure cached pixmap has correct DPR

Nicolas Fellarequested to merge
work/nico/cachye into master

Changing the devicePixelRatio on an existing pixmap can be expensive since it involves a detach and deep copy.

To avoid that we always want to have the cached pixmap to have the correct DPR. Use new KImageCache API to ensure that. Needs kguiaddons!224 (merged)

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