Fix preview widget appearing to scale twice

Summary:
It worked when I wrote it against integer scaling, it got broken when
fractional support was added, resulting in it rendering at the wrong
size when the host scaling is non integer.

BUG: 388218

Test Plan:
Ran with QT_SCALE_FACTOR=1.5 to simulate the host being at that scale
Opened preview and moved slider
At 1.0 buttons were smaller than the main KCM
At 1.5 buttons looked the same
At 2.0 looked bigger

Reviewers: broulik

Reviewed By: broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D14338