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 length = maskData.length;
    const scale = 1 / 255;
    for (let i = 3; i < length; i += 4) {
      const alpha = transferMap ? transferMap[maskData[i]] : maskData[i];
      layerData[i] = (layerData[i] * alpha * scale) | 0;
    }
  }
  function composeSMaskLuminosity(maskData, layerData, transferMap) {
    const length = maskData.length;
    for (let i = 3; i < length; i += 4) {
      const y =
        maskData[i - 3] * 77 + 
        maskData[i - 2] * 152 + 
        maskData[i - 1] * 28; 
      layerData[i] = transferMap
        ? (layerData[i] * transferMap[y >> 8]) >> 8
        : (layerData[i] * y) >> 16;
    }
  }
  function genericComposeSMask(
    maskCtx,
    layerCtx,
    width,