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 ctx = this.ctx;
    const current = this.current;
    const font = current.font;
    const fontSize = current.fontSize;
    const fontDirection = current.fontDirection;
    const spacingDir = font.vertical ? 1 : -1;
    const charSpacing = current.charSpacing;
    const wordSpacing = current.wordSpacing;
    const textHScale = current.textHScale * fontDirection;
    const fontMatrix = current.fontMatrix || FONT_IDENTITY_MATRIX;
    const glyphsLength = glyphs.length;
    const isTextInvisible =
      current.textRenderingMode === TextRenderingMode.INVISIBLE;
    let i, glyph, width, spacingLength;
    if (isTextInvisible || fontSize === 0) {
      return;
    }
    this._cachedGetSinglePixelWidth = null;
    ctx.save();
    ctx.transform.apply(ctx, current.textMatrix);
    ctx.translate(current.x, current.y);
    ctx.scale(textHScale, fontDirection);
    for (i = 0; i < glyphsLength; ++i) {
      glyph = glyphs[i];