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):

positions[i + 1],
      ]);
      const [x, y] = Util.applyTransform([0, 0], trans);
      ctx.drawImage(mask.canvas, x, y);
    }
    ctx.restore();
    this.compose();
  }
  paintImageMaskXObjectGroup(images) {
    if (!this.contentVisible) {
      return;
    }
    const ctx = this.ctx;
    const fillColor = this.current.fillColor;
    const isPatternFill = this.current.patternFill;
    for (let i = 0, ii = images.length; i < ii; i++) {
      const image = images[i];
      const width = image.width,
        height = image.height;
      const maskCanvas = this.cachedCanvases.getCanvas(
        "maskCanvas",
        width,
        height
      );
      const maskCtx = maskCanvas.context;
      maskCtx.save();
      putBinaryImageMask(maskCtx, image);
      maskCtx.globalCompositeOperation = "source-in";
      maskCtx.fillStyle = isPatternFill
        ? fillColor.getPattern(