Fix font rendering for non-HiDPI and integer scale factors

Summary:
QQC2-desktop-style was attempting to force the use of `Text.QtRendering` for non-integer scale factors. However, it was accidentally using it everywhere. This patch resolves the issue, and makes it use `Text.QtRendering` only for non-integer scale factors.

BUG: 391780
FIXED-IN: 5.45

Test Plan:
No visual changes when there is a non-integer scale factor.

System Settings before, no scale factor:
{F5751414}

System Settings after, no scale factor:
{F5751415}

Discover before, no scale factor:
{F5751416}

Discover after, no scale factor:
{F5751418}

To notice the differences, you will probably have to open them in different tabs, zoom in with {key Meta +}, and switch between the tabs.

Reviewers: mart, davidedmundson, broulik

Reviewed By: mart, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11274