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)

const coords = mesh.coords;
    const colors = mesh.colors;
    const ps = new Int32Array(16); 
    const cs = new Int32Array(4); 
    while (reader.hasData) {
      const f = reader.readFlag();
      if (!(0 <= f && f <= 3)) {
        throw new FormatError("Unknown type7 flag");
      }
      const pi = coords.length;
      for (let i = 0, ii = f !== 0 ? 12 : 16; i < ii; i++) {
        coords.push(reader.readCoordinate());
      }
      const ci = colors.length;
      for (let i = 0, ii = f !== 0 ? 2 : 4; i < ii; i++) {
        colors.push(reader.readComponents());
      }
      let tmp1, tmp2, tmp3, tmp4;
      switch (f) {
        
        case 0:
          ps[12] = pi + 3; ps[13] = pi + 4;  ps[14] = pi + 5;  ps[15] = pi + 6;
          ps[ 8] = pi + 2; ps[ 9] = pi + 13; ps[10] = pi + 14; ps[11] = pi + 7;
          ps[ 4] = pi + 1; ps[ 5] = pi + 12; ps[ 6] = pi + 15; ps[ 7] = pi + 8;