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 width = canvas.width;
      const height = canvas.height;
      const bl = Util.applyTransform([0, 0], inv);
      const br = Util.applyTransform([0, height], inv);
      const ul = Util.applyTransform([width, 0], inv);
      const ur = Util.applyTransform([width, height], inv);
      const x0 = Math.min(bl[0], br[0], ul[0], ur[0]);
      const y0 = Math.min(bl[1], br[1], ul[1], ur[1]);
      const x1 = Math.max(bl[0], br[0], ul[0], ur[0]);
      const y1 = Math.max(bl[1], br[1], ul[1], ur[1]);
      this.ctx.fillRect(x0, y0, x1 - x0, y1 - y0);
    } else {
      
      
      
      
      
      this.ctx.fillRect(-1e10, -1e10, 2e10, 2e10);
    }
    this.compose(this.current.getClippedPathBoundingBox());
    this.restore();
  }
  
  beginInlineImage() {
    unreachable("Should not call beginInlineImage");
  }
  beginImageData() {
    unreachable("Should not call beginImageData");
  }
  paintFormXObjectBegin(matrix, bbox) {