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):

maskToCanvas = Util.transform(maskToCanvas, [1, 0, 0, 1, 0, -height]);
    const cord1 = Util.applyTransform([0, 0], maskToCanvas);
    const cord2 = Util.applyTransform([width, height], maskToCanvas);
    const rect = Util.normalizeRect([cord1[0], cord1[1], cord2[0], cord2[1]]);
    const drawnWidth = Math.ceil(rect[2] - rect[0]);
    const drawnHeight = Math.ceil(rect[3] - rect[1]);
    const fillCanvas = this.cachedCanvases.getCanvas(
      "fillCanvas",
      drawnWidth,
      drawnHeight,
      true
    );
    const fillCtx = fillCanvas.context;
    
    const offsetX = Math.min(cord1[0], cord2[0]);
    const offsetY = Math.min(cord1[1], cord2[1]);
    fillCtx.translate(-offsetX, -offsetY);
    fillCtx.transform.apply(fillCtx, maskToCanvas);
    
    const scaled = this._scaleImage(
      maskCanvas.canvas,
      fillCtx.mozCurrentTransformInverse
    );
    fillCtx.imageSmoothingEnabled = getImageSmoothingEnabled(
      fillCtx.mozCurrentTransform,
      img.interpolate
    );
    fillCtx.drawImage(
      scaled.img,
      0,