Fix blurry icons by adding UseHighDpiPixmaps flag

Summary: When using fractional scaling, icons on the top are blurred. This patch fixes the issue.

Test Plan: No side effects.

Reviewers: KDE Games, yurchor

Reviewed By: yurchor

Subscribers: yurchor, kde-games-devel

Differential Revision: https://phabricator.kde.org/D29585