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):

const color = Util.makeHexColor(r, g, b);
    this.ctx.strokeStyle = color;
    this.current.strokeColor = color;
  }
  setFillRGBColor(r, g, b) {
    const color = Util.makeHexColor(r, g, b);
    this.ctx.fillStyle = color;
    this.current.fillColor = color;
    this.current.patternFill = false;
  }
  _getPattern(objId, matrix = null) {
    let pattern;
    if (this.cachedPatterns.has(objId)) {
      pattern = this.cachedPatterns.get(objId);
    } else {
      pattern = getShadingPattern(
        this.objs.get(objId),
        this.cachedCanvasPatterns
      );
      this.cachedPatterns.set(objId, pattern);
    }
    if (matrix) {
      pattern.matrix = matrix;
    }
    return pattern;
  }
  shadingFill(objId) {