Attach (recommended) or Link to PDF file here:
[Testfile v1.5 to PDFA2.pdf](https://github.com/mozilla/pdf.js/files/6310752/Testfile.v1.5.to.PDFA2.pdf)


Configuration:
- Web browser and its version: Version 89.0.4389.114 (Offizieller Build) (x86_64)
- Operating system and its version: Windows 10 / MacOS BigSur
- PDF.js version: 2.5.207
- Is a browser extension: no 

Steps to reproduce the problem:
1.  Open Testfile v1.5 to PDFA2.pdf in pdf.js viewer demo
2. text is covered up by markings / highlights which are stored in pdf itself

What is the expected behavior? (add screenshot)
Should be displayed as in Acrobat Reader
<img width="1169" alt="Bildschirmfoto 2021-04-14 um 14 24 41" src="https://user-images.githubusercontent.com/1286150/114710202-bf175a80-9d2d-11eb-8d54-75481fe91b7d.png">

What went wrong? (add screenshot)
It's how it is displayed in pdf.js viewer
<img width="782" alt="Bildschirmfoto 2021-04-14 um 14 24 16" src="https://user-images.githubusercontent.com/1286150/114710311-d8200b80-9d2d-11eb-8000-b67b94488929.png">

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

line = font.encodeString(line).join("");
    const glyphs = font.charsToGlyphs(line);
    if (glyphs.length <= 1) {
      
      return [line];
    }
    const positions = font.getCharPositions(line);
    const scale = fontSize / 1000;
    const chunks = [];
    let lastSpacePosInStringStart = -1,
      lastSpacePosInStringEnd = -1,
      lastSpacePos = -1,
      startChunk = 0,
      currentWidth = 0;
    for (let i = 0, ii = glyphs.length; i < ii; i++) {
      const [start, end] = positions[i];
      const glyph = glyphs[i];
      const glyphWidth = glyph.width * scale;
      if (glyph.unicode === " ") {
        if (currentWidth + glyphWidth > width) {
          
          chunks.push(line.substring(startChunk, start));
          startChunk = start;
          currentWidth = glyphWidth;
          lastSpacePosInStringStart = -1;
          lastSpacePos = -1;
        } else {
          currentWidth += glyphWidth;