Attach (recommended) or Link to PDF file here:
[PATTYP2.pdf](https://github.com/mozilla/pdf.js/files/6471935/PATTYP2.pdf)

Configuration:
- Web browser and its version: FF 88.0.1
- Operating system and its version: W10.0.19042.985
- PDF.js version: https://mozilla.github.io/pdf.js/web/viewer.html

Steps to reproduce the problem:
1. open file to display

What is the expected behavior? (add screenshot)
![grafik](https://user-images.githubusercontent.com/6665575/118114800-5e407800-b3e8-11eb-860f-019f48a8eabf.png)

What went wrong? (add screenshot)
![grafik](https://user-images.githubusercontent.com/6665575/118114841-6c8e9400-b3e8-11eb-8252-1517e5d76475.png)

(memo for me: when fixed, check the files in https://issues.apache.org/jira/browse/PDFBOX-1958 )

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) {