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 k;
    if (y < y1) {
      k = 0;
    } else if (y > y3) {
      k = 1;
    } else {
      k = (y1 - y) / (y1 - y3);
    }
    xb = x1 - (x1 - x3) * k;
    cbr = c1r - (c1r - c3r) * k;
    cbg = c1g - (c1g - c3g) * k;
    cbb = c1b - (c1b - c3b) * k;
    const x1_ = Math.round(Math.min(xa, xb));
    const x2_ = Math.round(Math.max(xa, xb));
    let j = rowSize * y + x1_ * 4;
    for (let x = x1_; x <= x2_; x++) {
      k = (xa - x) / (xa - xb);
      if (k < 0) {
        k = 0;
      } else if (k > 1) {
        k = 1;
      }
      bytes[j++] = (car - (car - cbr) * k) | 0;
      bytes[j++] = (cag - (cag - cbg) * k) | 0;
      bytes[j++] = (cab - (cab - cbb) * k) | 0;
      bytes[j++] = 255;
    }
  }
}
function drawFigure(data, figure, context) {