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):

let canvasEntry;
    if (this.cache[id] !== undefined) {
      canvasEntry = this.cache[id];
      this.canvasFactory.reset(canvasEntry, width, height);
      
      canvasEntry.context.setTransform(1, 0, 0, 1, 0, 0);
    } else {
      canvasEntry = this.canvasFactory.create(width, height);
      this.cache[id] = canvasEntry;
    }
    if (trackTransform) {
      addContextCurrentTransform(canvasEntry.context);
    }
    return canvasEntry;
  }
  clear() {
    for (const id in this.cache) {
      const canvasEntry = this.cache[id];
      this.canvasFactory.destroy(canvasEntry);
      delete this.cache[id];
    }
  }
}
/**
 * Least recently used cache implemented with a JS Map. JS Map keys are ordered
 * by last insertion.
 */
class LRUCache {