Make QRubberBand transparent on Mac OS X

Restore Q_WS_MAC code path.

Task-number: QTBUG-35987
Change-Id: I1ad50dd93e9c7116963e09d94ad114bcb6a1c31f
Reviewed-by: Liang Qi <liang.qi@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>