Attach (recommended) or Link to PDF file here: [A3_Quer.pdf](https://github.com/mozilla/pdf.js/files/7373152/A3_Quer.pdf)


**Configuration:**

- Web browser and its version: Firefox 93.0
- Operating system and its version: macOS 11.6
- PDF.js version: 2.5.207
- Is a browser extension: no

**Steps to reproduce the problem:**
1. Open the attached file in Firefox

We discovered this issue in on older version of this library, but it can be reproduced in the current version of Firefox.

**What is the expected behavior? (add screenshot)**
The file should be rendered with a linear gradient as a background:
![Bildschirmfoto 2021-10-19 um 14 34 00](https://user-images.githubusercontent.com/8649002/137910027-07947e34-700b-4692-b225-d38c36da0d37.png)


**What went wrong? (add screenshot)**
The file is rendered without any background:
![Bildschirmfoto 2021-10-19 um 14 35 05](https://user-images.githubusercontent.com/8649002/137910215-a6b8fb9e-ff95-49f0-93b4-c9f127bd49c1.png)



**Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension):**
none
throw new FormatError("Unknown type4 flag");
        }
        switch (f) {
          case 0:
            verticesLeft = 3;
            break;
          case 1:
            ps.push(ps.at(-2), ps.at(-1));
            verticesLeft = 1;
            break;
          case 2:
            ps.push(ps.at(-3), ps.at(-1));
            verticesLeft = 1;
            break;
        }
        operators.push(f);
      }
      ps.push(coords.length);
      coords.push(coord);
      colors.push(color);
      verticesLeft--;
      reader.align();
    }
    this.figures.push({
      type: "triangles",
      coords: new Int32Array(ps),
      colors: new Int32Array(ps),
    });
  }
  _decodeType5Shading(reader, verticesPerRow) {