Attach (recommended) or Link to PDF file here:
[shading_pattern.pdf](https://github.com/mozilla/pdf.js/files/1532598/shading_pattern.pdf)

Configuration:
- Web browser and its version: Firefox 57.0
- Operating system and its version: Debian GNU/Linux 9 64-bit
- PDF.js version: tried with Firefox's 1.9.583 and with online viewer 2.0.185
- Is a browser extension: no

Steps to reproduce the problem:
1. Open the attached file with the online viewer https://mozilla.github.io/pdf.js/web/viewer.html

What is the expected behavior? (add screenshot)
![image](https://user-images.githubusercontent.com/11061462/33626267-d9a57dee-d9d8-11e7-8712-cfa17c038259.png)
(Image captured from Adobe Reader CS.) The shading needs to be inside the letters.

What went wrong? (add screenshot)
![image](https://user-images.githubusercontent.com/11061462/33626285-eaabd462-d9d8-11e7-94e2-96480b57750d.png)
The shading seems to be displaced and upside-down, for it does not cover all the characters of the word.

Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension):

bold = "900";
    } else if (fontObj.bold) {
      bold = "bold";
    }
    const italic = fontObj.italic ? "italic" : "normal";
    const typeface = `"${name}", ${fontObj.fallbackName}`;
    
    
    
    
    let browserFontSize = size;
    if (size < MIN_FONT_SIZE) {
      browserFontSize = MIN_FONT_SIZE;
    } else if (size > MAX_FONT_SIZE) {
      browserFontSize = MAX_FONT_SIZE;
    }
    this.current.fontSizeScale = size / browserFontSize;
    this.ctx.font = `${italic} ${bold} ${browserFontSize}px ${typeface}`;
  }
  setTextRenderingMode(mode) {
    this.current.textRenderingMode = mode;
  }
  setTextRise(rise) {
    this.current.textRise = rise;
  }
  moveText(x, y) {
    this.current.x = this.current.lineX += x;
    this.current.y = this.current.lineY += y;
  }
  setLeadingMoveText(x, y) {