Fix blurry icons in HiDPI

Summary: BUG: 390903

Test Plan:
Looks great in HiDPI now. With scale factor 1.2, for example:
{F5727961}

Reviewers: #konqueror, wbauer, stikonas

Reviewed By: stikonas

Differential Revision: https://phabricator.kde.org/D10794