function dotOffPointTarget(contour, offIndex) {
  let prevOn, nextOn;
  if (contour[mod(offIndex-1, contour.length)].on) 
    prevOn = tuple(contour[mod(offIndex-1, contour.length)]);
  else prevOn = tuple(contour[mod(offIndex-2, contour.length)]);
  if (contour[mod(offIndex+1, contour.length)].on) 
    nextOn = tuple(contour[mod(offIndex+1, contour.length)]);
  else nextOn = tuple(contour[mod(offIndex+2, contour.length)]);
  return projection(tuple(contour[offIndex]), prevOn, nextOn);
}
Get the target point of a dot's off-point
@param { Contour } contour 
@param { number } offIndex 
@returns { [ number, number ] }