Attach (recommended) or Link to PDF file here:
[output.pdf](https://github.com/mozilla/pdf.js/files/6374987/output.pdf)

Configuration:
- Web browser and its version: Firefox 88.0
- Operating system and its version: macOS Big Sur 11.2.3 (20D91)
- PDF.js version: 2.8.243
- Is a browser extension: no/Firefox built-in

Steps to reproduce the problem:
1.  add Ink-Annotations to a pdf (i use https://github.com/highkite/pdfAnnotate, Annotations with appearance streams are shown correctly)
2. open output file in firefox

What is the expected behavior? 
The annotations should be visible.
![Bildschirmfoto 2021-04-26 um 09 15 48](https://user-images.githubusercontent.com/33630884/116043809-6a020f80-a670-11eb-973c-6f2df3dae532.png)

What went wrong? 
The annotations are added to the dom but invisible.
![Bildschirmfoto 2021-04-26 um 09 18 26](https://user-images.githubusercontent.com/33630884/116043867-7e460c80-a670-11eb-964d-0cc2c6dbfb61.png)

Tested successfull with FoxitReader, AcrobatReader, Chrome and Safari.
const xMid = x0 + (x1 - x0) / 2;
          const yMid = y0 + (y1 - y0) / 2;
          const xOffset = ((x1 - x0) / 2) * controlPointsDistance;
          const yOffset = ((y1 - y0) / 2) * controlPointsDistance;
          buffer.push(`${xMid} ${y1} m`);
          buffer.push(
            `${xMid + xOffset} ${y1} ${x1} ${yMid + yOffset} ${x1} ${yMid} c`
          );
          buffer.push(
            `${x1} ${yMid - yOffset} ${xMid + xOffset} ${y0} ${xMid} ${y0} c`
          );
          buffer.push(
            `${xMid - xOffset} ${y0} ${x0} ${yMid - yOffset} ${x0} ${yMid} c`
          );
          buffer.push(
            `${x0} ${yMid + yOffset} ${xMid - xOffset} ${y1} ${xMid} ${y1} c`
          );
          buffer.push("h");
          if (fillColor) {
            buffer.push("B");
          } else {
            buffer.push("S");
          }
          return [points[0].x, points[1].x, points[3].y, points[1].y];
        },
      });
    }
  }
}
class PolylineAnnotation extends MarkupAnnotation {