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):

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      const m = this.ctx.mozCurrentTransform;
      const absDet = Math.abs(m[0] * m[3] - m[2] * m[1]);
      const sqNorm1 = m[0] ** 2 + m[2] ** 2;
      const sqNorm2 = m[1] ** 2 + m[3] ** 2;
      const pixelHeight = Math.sqrt(Math.max(sqNorm1, sqNorm2)) / absDet;
      if (sqNorm1 !== sqNorm2 && this._combinedScaleFactor * pixelHeight > 1) {
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        this._cachedGetSinglePixelWidth = -(
          this._combinedScaleFactor * pixelHeight
        );
      } else if (absDet > Number.EPSILON) {
        this._cachedGetSinglePixelWidth = pixelHeight;
      } else {
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        this._cachedGetSinglePixelWidth = 1;
      }
    }
    return this._cachedGetSinglePixelWidth;
  }
  getCanvasPosition(x, y) {