Add offset reading support in pnghandler

This patch adds the offset reading that matches the offset writing in
writeImage

Qt 5 fix commit 0b62a5d3c931aa4d91b986ec518567e9463a6360

Task-number: QTBUG-32674
Change-Id: I64f7e31049834ecbd052a061b79d3ed97e60e063
Reviewed-by: aavit <eirik.aavitsland@digia.com>
Reviewed-by: Laszlo Papp <lpapp@kde.org>