[Logout screen] Fix label opacity failing

Summary: Due to Qt bug: https://bugreports.qt.io/browse/QTBUG-70138 (KDE bug: https://bugs.kde.org/show_bug.cgi?id=401644) we have label opacity utterly failing when the logout screen is superimposed over a bright background. Even though QtRendering is the inferior renderer, this patch uses it as a workaround for the aforementioned bug since ugly (if it ever would be) text is better than (almost) no text.

Test Plan:
Before:

After:

Reviewers: Plasma, ngraham, rooty

Reviewed By: ngraham, rooty

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D19510