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 )

const length = bytes.length;
    for (let i = 3; i < length; i += 4) {
      const alpha = bytes[i];
      if (alpha === 0) {
        bytes[i - 3] = r0;
        bytes[i - 2] = g0;
        bytes[i - 1] = b0;
      } else if (alpha < 255) {
        const alpha_ = 255 - alpha;
        bytes[i - 3] = (bytes[i - 3] * alpha + r0 * alpha_) >> 8;
        bytes[i - 2] = (bytes[i - 2] * alpha + g0 * alpha_) >> 8;
        bytes[i - 1] = (bytes[i - 1] * alpha + b0 * alpha_) >> 8;
      }
    }
  }
  function composeSMaskAlpha(maskData, layerData, transferMap) {