Convert bounding box points from (width,height,center) to (height,width,top,left).

def bb2hw(a:Collection[int])->np.ndarray:
    "Convert bounding box points from (width,height,center) to (height,width,top,left)."
    return np.array([a[1],a[0],a[3]-a[1],a[2]-a[0]])