Attach (recommended) or Link to PDF file here:
[Erstattung von Arbeitgeberaufwendungen nach __ 56 Abs. 1 und 57, 58 Infektionsschutzgesetz - Antrag(1).pdf](https://github.com/mozilla/pdf.js/files/6009589/Erstattung.von.Arbeitgeberaufwendungen.nach.__.56.Abs.1.und.57.58.Infektionsschutzgesetz.-.Antrag.1.pdf)

Also available at: [formularserver.bayern.de](https://formularserver.bayern.de/intelliform/forms/stmi+regierungen/rof/b5/55.2/rof_55.2-086/index)

Configuration:
- Web browser and its version: FF 85.0.2 and FF 86.0b9
- Operating system and its version: Win 10
- PDF.js version: 
FF 85.0.2:
const pdfjsVersion = '2.7.345';
const pdfjsBuild = 'b194c820b';

FF 86.0b9:
const pdfjsVersion = '2.7.510';
const pdfjsBuild = '631bada0d';

- Is a browser extension:
no
Steps to reproduce the problem:
1. load pdf document

What is the expected behavior? (add screenshot)
This is with latest Chrome:
![grafik](https://user-images.githubusercontent.com/584660/108499622-e2360880-72ae-11eb-82a7-b181619ff079.png)


What went wrong? (add screenshot)
The checkboxes for e.g. "Geschlecht" are black and you cannot see which one is checked. But this is true for every checkbox in that document.
![grafik](https://user-images.githubusercontent.com/584660/108499753-14e00100-72af-11eb-89d3-1f5ca652e058.png)

Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension):

fillAlpha,
        pointsCallback: (buffer, points) => {
          const x0 = points[0].x + this.borderStyle.width / 2;
          const y0 = points[0].y - this.borderStyle.width / 2;
          const x1 = points[3].x - this.borderStyle.width / 2;
          const y1 = points[3].y + this.borderStyle.width / 2;
          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} ${y1} m`,
            `${xMid + xOffset} ${y1} ${x1} ${yMid + yOffset} ${x1} ${yMid} c`,
            `${x1} ${yMid - yOffset} ${xMid + xOffset} ${y0} ${xMid} ${y0} c`,
            `${xMid - xOffset} ${y0} ${x0} ${yMid - yOffset} ${x0} ${yMid} c`,
            `${x0} ${yMid + yOffset} ${xMid - xOffset} ${y1} ${xMid} ${y1} c`,
            "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 {