Fix blocky text on splash screen when using non-integer scale factor

Summary: The Qt bug that makes Text.NativeRendering look terrible with non-integer scale factors (https://bugreports.qt.io/browse/QTBUG-67007) is still a problem, so we need to apply the same workaround here that we previously applied to all the PlasmaComponents.

Test Plan:
No visual change at 1x scale. Looks much better at 1.5x scale.

1.5x scale, before:

1.5x scale, after:

Reviewers: Plasma, davidedmundson, broulik

Reviewed By: Plasma, davidedmundson, broulik

Subscribers: broulik, svenmauch, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D14347