Fix high-dpi QGraphicsPixmapItem bounding rects.

QGraphicsPixmapItem did not take the pixmaps
devicePixelRatio into account when calculating the
bounding rect. As a consequence, a 512x512@2x pixmap
would get a 512x512 bounding size instead of the
correct 256x256 bounding size.

Task-number: QTBUG-37008
Change-Id: I266e418b7e5d92bf1d4c96bd9380a27708dc2b4a
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: Andreas Aardal Hanssen <andreas@hanssen.name>
Cocoa: Set "Stereo" OpenGL format option.

Task-number: QTBUG-35548

Change-Id: I60b633ae44b8b4b353a7328eb0922b9301e1460c
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>