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):

metrics = {
        width: 0.791 * fontSize,
        height: 0.705 * fontSize,
      };
      char = "\x6C";
    } else {
      unreachable(`_getDefaultCheckedAppearance - unsupported type: ${type}`);
    }
    
    const xShift = (width - metrics.width) / 2;
    const yShift = (height - metrics.height) / 2;
    const appearance = `q BT /PdfJsZaDb ${fontSize} Tf 0 g ${xShift} ${yShift} Td (${char}) Tj ET Q`;
    const appearanceStreamDict = new Dict(params.xref);
    appearanceStreamDict.set("FormType", 1);
    appearanceStreamDict.set("Subtype", Name.get("Form"));
    appearanceStreamDict.set("Type", Name.get("XObject"));
    appearanceStreamDict.set("BBox", bbox);
    appearanceStreamDict.set("Matrix", [1, 0, 0, 1, 0, 0]);
    appearanceStreamDict.set("Length", appearance.length);
    const resources = new Dict(params.xref);
    const font = new Dict(params.xref);
    font.set("PdfJsZaDb", this.fallbackFontDict);
    resources.set("Font", font);
    appearanceStreamDict.set("Resources", resources);
    this.checkedAppearance = new StringStream(appearance);
    this.checkedAppearance.dict = appearanceStreamDict;
    this._streams.push(this.checkedAppearance);
  }
  _processCheckBox(params) {