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 (typeof ImageData !== "undefined" && imgData instanceof ImageData) {
    ctx.putImageData(imgData, 0, 0);
    return;
  }
  
  
  
  
  
  
  
  
  
  
  const height = imgData.height,
    width = imgData.width;
  const partialChunkHeight = height % FULL_CHUNK_HEIGHT;
  const fullChunks = (height - partialChunkHeight) / FULL_CHUNK_HEIGHT;
  const totalChunks = partialChunkHeight === 0 ? fullChunks : fullChunks + 1;
  const chunkImgData = ctx.createImageData(width, FULL_CHUNK_HEIGHT);
  let srcPos = 0,
    destPos;
  const src = imgData.data;
  const dest = chunkImgData.data;
  let i, j, thisChunkHeight, elemsInThisChunk;
  let transferMapRed, transferMapGreen, transferMapBlue, transferMapGray;
  if (transferMaps) {
    switch (transferMaps.length) {
      case 1:
        transferMapRed = transferMaps[0];