shortenFrom(by) {
    let f = Vector2.subtract(this.to, this.from);
    f.normalize();
    f.multiplyScalar(by);
    this.from.add(f);
    return this;
  }
Shortens this line from the "from" direction by a given value (in pixels).

@param {Number} by The length in pixels to shorten the vector by.
@returns {Line} This line.