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.
let pattern;
    if (this.cachedPatterns.has(objId)) {
      pattern = this.cachedPatterns.get(objId);
    } else {
      pattern = getShadingPattern(this.objs.get(objId));
      this.cachedPatterns.set(objId, pattern);
    }
    if (matrix) {
      pattern.matrix = matrix;
    }
    return pattern;
  }
  shadingFill(objId) {
    if (!this.contentVisible) {
      return;
    }
    const ctx = this.ctx;
    this.save();
    const pattern = this._getPattern(objId);
    ctx.fillStyle = pattern.getPattern(
      ctx,
      this,
      ctx.mozCurrentTransformInverse,
      PathType.SHADING
    );
    const inv = ctx.mozCurrentTransformInverse;
    if (inv) {
      const canvas = ctx.canvas;