cv.approxPolyDP maxContour.

[TomAtRichfords]

During testing, I encountered a case where the top-right of my page was incorrectly evaluated as being near the top-left, resulting in a near triangular poly from getCornerPoints(contour). What seems to have happened is that there was a point in the contour that was just right of the centre, yet had a bigger distance than the right-hand corner of the page (although the page was at an almost 45 degree angle in the frame, which would be unusual). Recommend applying a cv.approxPolyDP to the maxContour prior to getCornerPoints(contour) to reduce occurrences.

      return this.getApproxPoly(maxContour);
    }

    /** refines a contour to get approximate poly
     * @param {*} contour contour to refine
     */
    getApproxPoly(contour) {
      const peri = cv.arcLength(contour, true);
      const approx = new cv.Mat();
      cv.arcLength(contour, true) * 0.02
      cv.approxPolyDP(contour, approx, 0.1 * peri, true);
      return approx;
    }