[REG: Qt5.6-alpha -> Qt5.6-beta] [OS X] imgBoundingRect doesn't have any effect

This worked fine with Qt5.4 and Qt5.6-alpha on both Windows and Mac OS X.
With Qt.5.6-beta, things still look fine on Windows, but on Mac OS X the
parameter imgBoundingRect does not seem to have any effect and the image is drawn at full resolution within the window, i.e. only upper left
portion of the image can be seen.

Use the attached test application to reproduce the issue.