offscreenquickview: Keep DPR up to date
When a screen scale changes dynamically, the underlying platform screen scale changes. The offscreen window screen scale does not change as it's backed by a separate render target with an explicitly set device pixel ratio.
This patch updates the device pixel ratio on each frame paint directly from the screen scale factor.
(cherry picked from commit 61e7dd97)
Co-authored-by: David Edmundson kde@davidedmundson.co.uk