Fix Invalid Drawable error for QGLWidget on Mac

You are not supposed to call NSOpenGLContext -setView: for a view that
has not yet called drawRect.  We we attempted to do this, we would get
the invalid drawable error, leading to QGLWidgets just drawing garbage.

Task-number: QTBUG-28175
Change-Id: I47aef07b4676f2db8591f98fc1661f6f447bdef9
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>