They were incorrectly attempted to be read as Indexed8, instead use
the RGBA64 formats to read them with full accuracy.
Task-number: QTBUG-45858
Change-Id: I14fc3cb0d59fa2fc0e68fd870f3b32352c34161f
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>