Flip a keypoint vertically around the x-axis.

def keypoint_vflip(kp, rows, cols):
    """Flip a keypoint vertically around the x-axis."""
    x, y, angle, scale = kp
    c = math.cos(angle)
    s = math.sin(angle)
    angle = math.atan2(-s, c)
    return [x, (rows - 1) - y, angle, scale]