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)

ps[q + verticesPerRow + 1],
              ps[q + 1],
              ps[q + verticesPerRow],
              cs[q + verticesPerRow + 1],
              cs[q + 1],
              cs[q + verticesPerRow]
            );
          }
        }
        break;
      case "triangles":
        for (i = 0, ii = ps.length; i < ii; i += 3) {
          drawTriangle(
            data,
            context,
            ps[i],
            ps[i + 1],
            ps[i + 2],
            cs[i],
            cs[i + 1],
            cs[i + 2]
          );
        }
        break;
      default:
        throw new Error("illegal figure");
    }
  }
  
  function createMeshCanvas(