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 minY = Math.round(y1),
    maxY = Math.round(y3);
  let xa, car, cag, cab;
  let xb, cbr, cbg, cbb;
  for (let y = minY; y <= maxY; y++) {
    if (y < y2) {
      let k;
      if (y < y1) {
        k = 0;
      } else {
        k = (y1 - y) / (y1 - y2);
      }
      xa = x1 - (x1 - x2) * k;
      car = c1r - (c1r - c2r) * k;
      cag = c1g - (c1g - c2g) * k;
      cab = c1b - (c1b - c2b) * k;
    } else {
      let k;
      if (y > y3) {
        k = 1;
      } else if (y2 === y3) {
        k = 0;
      } else {
        k = (y2 - y) / (y2 - y3);
      }
      xa = x2 - (x2 - x3) * k;
      car = c2r - (c2r - c3r) * k;
      cag = c2g - (c2g - c3g) * k;
      cab = c2b - (c2b - c3b) * k;
    }