Fix pixmap scaling on HiDPI screens

Summary:
This fixes the pixelation seen when using display scaling on HiDPI screens. This fixes the main screen as well as the settings and other areas.

BUG: 390081

Before:
{F5765495}

After:
{F5765496}

Test Plan: Browse file structure in project, open settings and change values, ensuring functionality.

Reviewers: aacid, lesliezhai, rkflx

Reviewed By: rkflx

Subscribers: rkflx, ngraham

Differential Revision: https://phabricator.kde.org/D11669