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)

throw new FormatError("Unknown type4 flag");
        }
        switch (f) {
          case 0:
            verticesLeft = 3;
            break;
          case 1:
            ps.push(ps[ps.length - 2], ps[ps.length - 1]);
            verticesLeft = 1;
            break;
          case 2:
            ps.push(ps[ps.length - 3], ps[ps.length - 1]);
            verticesLeft = 1;
            break;
        }
        operators.push(f);
      }
      ps.push(coords.length);
      coords.push(coord);
      colors.push(color);
      verticesLeft--;
      reader.align();
    }
    mesh.figures.push({
      type: "triangles",
      coords: new Int32Array(ps),
      colors: new Int32Array(ps),
    });
  }
  function decodeType5Shading(mesh, reader, verticesPerRow) {