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):

throw new Error("Bounding box is required.");
    }
    
    
    let bounds = Util.getAxialAlignedBoundingBox(
      group.bbox,
      currentCtx.mozCurrentTransform
    );
    
    const canvasBounds = [
      0,
      0,
      currentCtx.canvas.width,
      currentCtx.canvas.height,
    ];
    bounds = Util.intersect(bounds, canvasBounds) || [0, 0, 0, 0];
    
    
    const offsetX = Math.floor(bounds[0]);
    const offsetY = Math.floor(bounds[1]);
    let drawnWidth = Math.max(Math.ceil(bounds[2]) - offsetX, 1);
    let drawnHeight = Math.max(Math.ceil(bounds[3]) - offsetY, 1);
    let scaleX = 1,
      scaleY = 1;
    if (drawnWidth > MAX_GROUP_SIZE) {
      scaleX = drawnWidth / MAX_GROUP_SIZE;
      drawnWidth = MAX_GROUP_SIZE;
    }
    if (drawnHeight > MAX_GROUP_SIZE) {
      scaleY = drawnHeight / MAX_GROUP_SIZE;