Add offset reading support in pnghandler

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

Task-number: QTBUG-32674
Change-Id: I264ba41163e59638d7219e0a8913f9d455830883
Reviewed-by: aavit <eirik.aavitsland@digia.com>
test: Mark tst_QSettings::ctor() and tst_QSettings::rainersSyncBugOnMac(native) as XFAIL

Mark some tests as expected failures on OS X 10.8

 - tst_QSettings::ctor(native)
 - tst_QSettings::ctor(ini)
 - tst_QSettings::ctor(custom1)
 - tst_QSettings::ctor(custom2)
 - tst_QSettings::rainersSyncBugOnMac(native)

Task-number: QTBUG-32655
Change-Id: I54928d991a8ccf300b40747feaa6fda9d124781b
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>