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)

height,
    subtype,
    backdrop,
    transferMap
  ) {
    const hasBackdrop = !!backdrop;
    const r0 = hasBackdrop ? backdrop[0] : 0;
    const g0 = hasBackdrop ? backdrop[1] : 0;
    const b0 = hasBackdrop ? backdrop[2] : 0;
    let composeFn;
    if (subtype === "Luminosity") {
      composeFn = composeSMaskLuminosity;
    } else {
      composeFn = composeSMaskAlpha;
    }
    
    const PIXELS_TO_PROCESS = 1048576;
    const chunkSize = Math.min(height, Math.ceil(PIXELS_TO_PROCESS / width));
    for (let row = 0; row < height; row += chunkSize) {
      const chunkHeight = Math.min(chunkSize, height - row);
      const maskData = maskCtx.getImageData(0, row, width, chunkHeight);
      const layerData = layerCtx.getImageData(0, row, width, chunkHeight);
      if (hasBackdrop) {
        composeSMaskBackdrop(maskData.data, r0, g0, b0);
      }
      composeFn(maskData.data, layerData.data, transferMap);
      maskCtx.putImageData(layerData, 0, row);
    }
  }
  function composeSMask(ctx, smask, layerCtx) {