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.
thisChunkHeight = partialChunkHeight;
        elemsInThisChunk = width * thisChunkHeight;
      }
      destPos = 0;
      for (j = elemsInThisChunk; j--; ) {
        dest[destPos++] = src[srcPos++];
        dest[destPos++] = src[srcPos++];
        dest[destPos++] = src[srcPos++];
        dest[destPos++] = 255;
      }
      if (hasTransferMaps) {
        for (let k = 0; k < destPos; k += 4) {
          if (transferMapRed) {
            dest[k + 0] = transferMapRed[dest[k + 0]];
          }
          if (transferMapGreen) {
            dest[k + 1] = transferMapGreen[dest[k + 1]];
          }
          if (transferMapBlue) {
            dest[k + 2] = transferMapBlue[dest[k + 2]];
          }
        }
      }
      ctx.putImageData(chunkImgData, 0, i * FULL_CHUNK_HEIGHT);
    }
  } else {
    throw new Error(`bad image kind: ${imgData.kind}`);
  }
}
function putBinaryImageMask(ctx, imgData) {