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 = img.width;
    const height = img.height;
    let widthScale = Math.max(
      Math.hypot(inverseTransform[0], inverseTransform[1]),
      1
    );
    let heightScale = Math.max(
      Math.hypot(inverseTransform[2], inverseTransform[3]),
      1
    );
    let paintWidth = width,
      paintHeight = height;
    let tmpCanvasId = "prescale1";
    let tmpCanvas, tmpCtx;
    while (
      (widthScale > 2 && paintWidth > 1) ||
      (heightScale > 2 && paintHeight > 1)
    ) {
      let newWidth = paintWidth,
        newHeight = paintHeight;
      if (widthScale > 2 && paintWidth > 1) {
        newWidth = Math.ceil(paintWidth / 2);
        widthScale /= paintWidth / newWidth;