fix stone image anti-aliasing issue in BoardPositionView
fix stone image anti-aliasing issue in BoardPositionView

herzbube/littlego,herzbube/littlego,herzbube/littlego,puremourning/littlego,herzbube/littlego,herzbube/littlego,puremourning/littlego,puremourning/littlego,puremourning/littlego