serialize([blX, blY, trX, trY], _rotation) {
    const outlines = [];
    const width = trX - blX;
    const height = trY - blY;
    for (const outline of this.#outlines) {
      const points = new Array(outline.length);
      for (let i = 0; i < outline.length; i += 2) {
        points[i] = blX + outline[i] * width;
        points[i + 1] = trY - outline[i + 1] * height;
      }
      outlines.push(points);
    }
    return outlines;
  }
Serialize the outlines into the PDF page coordinate system.
@param {Array<number>} _bbox - the bounding box of the annotation.
@param {number} _rotation - the rotation of the annotation.
@returns {Array<Array<number>>}