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):

this.xobjs = null;
    this.commonObjs = commonObjs;
    this.objs = objs;
    this.canvasFactory = canvasFactory;
    this.imageLayer = imageLayer;
    this.groupStack = [];
    this.processingType3 = null;
    
    
    this.baseTransform = null;
    this.baseTransformStack = [];
    this.groupLevel = 0;
    this.smaskStack = [];
    this.smaskCounter = 0;
    this.tempSMask = null;
    this.suspendedCtx = null;
    this.contentVisible = true;
    this.markedContentStack = [];
    this.optionalContentConfig = optionalContentConfig;
    this.cachedCanvases = new CachedCanvases(this.canvasFactory);
    this.cachedCanvasPatterns = new LRUCache(MAX_CACHED_CANVAS_PATTERNS);
    this.cachedPatterns = new Map();
    if (canvasCtx) {
      
      
      addContextCurrentTransform(canvasCtx);
    }
    this._cachedGetSinglePixelWidth = null;
  }
  beginDrawing({