PreviewJob : Support for DeviceRatioPixel
Summary: Allow users of KPreviewJob to request a devicePixelRatio for generated thumbnails. The result image is not guaranteed to have this ratio, and users should paint it accordingly.
Adds a convenient setDefaultDevicePixelRatio
to allow an application to set its preferred device pixel ratio with a one liner patch. See dolphin's system/dolphin!147 (merged)
The images using a pixel ratio of 2 are stored in cache in ~/.cache/thumbnails/large for 256x256 (i.e 128128@ logical size) and ~/.cache/thumbnails/x-large for 512x512 (i.e 256x256 logical size) and xx-large for 1024x1024 (i.e 512x512 logical size)
Follows xdg thumbnail specs 0.9 https://specifications.freedesktop.org/thumbnail-spec/thumbnail-spec-latest.html
Prior work: https://phabricator.kde.org/D29397
Needed for network/kio-extras!62 (merged) and system/dolphin!147 (merged)