[KStandardItemListWidget] Round to icon size before applying scaling

Otherwise for 22px icons on high dpi we will request either the wrong icons or scale them needlessly.
Dolphin requests a pixmap of 22px, this is then multiplied by dpr resulting in 44 which isn't a valid icon size (only 32 or 48 are).
Moreover, we will also hit the path where it will scale the pixmap to a proper icon size resulting in blurry icons
(and a performance penalty).

Differential Revision: https://phabricator.kde.org/D15260