Fix highdpi CE_HeaderLabel pixmaps in fusion style

Make sure enough pixels is sourced by multiplying
the layout rect size with the pixmaps devicePixelRatio.

Task-number: QTBUG-31477

Change-Id: I742603c09a65a520587da3eef9479ef7f0c87022
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>