This is a regression from #12864. The underscore is now too thin when the document is zoomed in.

URL: https://github.com/mozilla/pdf.js/files/2321540/issue6127.pdf
PDF.js 2.7.526: https://i.imgur.com/T4mIpct.png
PDF.js 2.7.510: https://i.imgur.com/nbwEpGT.png
Adobe Acrobat Reader DC 2020.013.20074: https://i.imgur.com/wDFBf3R.png

The screenshots were taken at 400% zoom.
if (isNum(glyph)) {
        spacingLength = (spacingDir * glyph * fontSize) / 1000;
        this.ctx.translate(spacingLength, 0);
        current.x += spacingLength * textHScale;
        continue;
      }
      const spacing = (glyph.isSpace ? wordSpacing : 0) + charSpacing;
      const operatorList = font.charProcOperatorList[glyph.operatorListId];
      if (!operatorList) {
        warn(`Type3 character "${glyph.operatorListId}" is not available.`);
        continue;
      }
      if (this.contentVisible) {
        this.processingType3 = glyph;
        this.save();
        ctx.scale(fontSize, fontSize);
        ctx.transform.apply(ctx, fontMatrix);
        this.executeOperatorList(operatorList);
        this.restore();
      }
      const transformed = Util.applyTransform([glyph.width, 0], fontMatrix);
      width = transformed[0] * fontSize + spacing;
      ctx.translate(width, 0);
      current.x += width * textHScale;
    }
    ctx.restore();
    this.processingType3 = null;
  }
  
  setCharWidth(xWidth, yWidth) {