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 (this.suspendedCtx) {
      throw new Error("beginSMaskMode called while already in smask mode");
    }
    const drawnWidth = this.ctx.canvas.width;
    const drawnHeight = this.ctx.canvas.height;
    const cacheId = "smaskGroupAt" + this.groupLevel;
    const scratchCanvas = this.cachedCanvases.getCanvas(
      cacheId,
      drawnWidth,
      drawnHeight,
      true
    );
    this.suspendedCtx = this.ctx;
    this.ctx = scratchCanvas.context;
    const ctx = this.ctx;
    ctx.setTransform.apply(ctx, this.suspendedCtx.mozCurrentTransform);
    copyCtxState(this.suspendedCtx, ctx);
    mirrorContextOperations(ctx, this.suspendedCtx);
    this.setGState([
      ["BM", "source-over"],
      ["ca", 1],
      ["CA", 1],
    ]);
  }
  endSMaskMode() {