Méven Car authored Mar 06, 2021 and David Faure committed Mar 06, 2021

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 device pixel ratio preferred with a one liner patch.

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