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.
const inSMaskMode = !!this.suspendedCtx;
    if (this.current.activeSMask && !inSMaskMode) {
      this.beginSMaskMode();
    } else if (!this.current.activeSMask && inSMaskMode) {
      this.endSMaskMode();
    }
    
  }
  /**
   * Soft mask mode takes the current main drawing canvas and replaces it with
   * a temporary canvas. Any drawing operations that happen on the temporary
   * canvas need to be composed with the main canvas that was suspended (see
   * `compose()`). The temporary canvas also duplicates many of its operations
   * on the suspended canvas to keep them in sync, so that when the soft mask
   * mode ends any clipping paths or transformations will still be active and in
   * the right order on the canvas' graphics state stack.
   */
  beginSMaskMode() {