Attach (recommended) or Link to PDF file here:

Configuration:
- Web browser and its version: Chrome 90.0.4430.93
- Operating system and its version: macOs Big Sur 11.2.3
- PDF.js version: 2.7.570
- Is a browser extension: No

Steps to reproduce the problem:
1. Load the attached pdf in https://mozilla.github.io/pdf.js/web/viewer.html
2. Go to page 46. Dotted lines are missing in the graphics (see screenshot from acrobat reader)

What is the expected behavior? (add screenshot) See the screenshot
[Technical_Guide_UK.pdf](https://github.com/mozilla/pdf.js/files/6413001/Technical_Guide_UK.pdf)
![Screenshot 2021-05-03 at 07 46 09](https://user-images.githubusercontent.com/3785784/116844541-a343fd80-abe3-11eb-94c3-d03b3dfd6769.png)

let xa, car, cag, cab;
    let xb, cbr, cbg, cbb;
    for (let y = minY; y <= maxY; y++) {
      if (y < y2) {
        let k;
        if (y < y1) {
          k = 0;
        } else if (y1 === y2) {
          k = 1;
        } else {
          k = (y1 - y) / (y1 - y2);
        }
        xa = x1 - (x1 - x2) * k;
        car = c1r - (c1r - c2r) * k;
        cag = c1g - (c1g - c2g) * k;
        cab = c1b - (c1b - c2b) * k;
      } else {
        let k;
        if (y > y3) {
          k = 1;
        } else if (y2 === y3) {
          k = 0;
        } else {
          k = (y2 - y) / (y2 - y3);
        }
        xa = x2 - (x2 - x3) * k;
        car = c2r - (c2r - c3r) * k;
        cag = c2g - (c2g - c3g) * k;
        cab = c2b - (c2b - c3b) * k;
      }