_UnsetWindowCorner
_UnsetWindowCorner(fLeftTopCrossing->RightBottomCorner());
_UnsetWindowCorner(fRightTopCrossing->LeftBottomCorner());
_UnsetWindowCorner(fLeftBottomCrossing->RightTopCorner());
_UnsetWindowCorner(fRightBottomCrossing->LeftTopCorner());
inline void _UnsetWindowCorner(Corner* corner);