Fix app icon button in window decos with high DPI in previews.

Summary:
With both Qt devicePixelRatio > 1 and Qt::AA_UseHighDpiPixmaps set
QIcon.pixmap() will return a bigger image than the size requested, with
the relevant devicePixelRatio set.

This code is trying to center an icon within a bounding rect, (like
QStyle::drawItemPixmap) and so positioning should use normalised pixels,
not actual pixmap pixels.

This affected Systemsettings previews but not kwin.

Reviewers: #plasma, mart

Reviewed By: #plasma, mart

Subscribers: mart, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D6763