Cocoa: avoid a crash in QCocoaDrag

NSEvent needs to be copied.

Reference:
http://lists.apple.com/archives/cocoa-dev/2007/Dec/msg00678.html

Task-number: QTBUG-33533
Change-Id: I73709545573e59aab6875a8c3dd903cb171e858f
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Make OpenGL examples work on retina displays.

glViewport expects device pixels, but the various
geometry accessors returns values in device-independent
pixels.

Change-Id: I8004692de82251e4f1f25bf8f2698895f222ede3
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>