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