Add support for HiDPi use case
Hidpi where the logical size does not correspond to the pixel on screen, needs appropriate thumbnails.
The proposed spec evolution specifies that Hidpi thumbnails might be generated in separate folders with "@2x" postfix added compared to the normal thumbnail folder.
This was first proposed for discussion on xdg ML https://lists.freedesktop.org/archives/xdg/2020-May/014316.html
An implementation is proposed for KDE Frameworks https://phabricator.kde.org/D29397