The function was only well defined from RGB32 and ARGB32PM formats,
this patch fixes it so it behaves well from all formats.
Task-number: QTBUG-63163
Change-Id: Id892531d9aaf997b707b430196c1166493792a2a
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>