Link to PDF file (or attach file here):
[pages4.pdf](https://github.com/mozilla/pdf.js/files/1128300/pages4.pdf)

Configuration:
- Web browser and its version: Firefox 54
- Operating system and its version: Win 10
- PDF.js version: 1.8.522
- Is an extension: no

What is the expected behavior? (add screenshot)
![chrome1-1 1](https://user-images.githubusercontent.com/146436/27918380-2f74512a-6245-11e7-8525-d0038fa6c2e8.jpg)
![chrome1-2 1](https://user-images.githubusercontent.com/146436/27918425-54f8e47e-6245-11e7-9bff-127f79ad0511.jpg)


What went wrong? (add screenshot)
![screenshot from 2017-07-06 12-10-30](https://user-images.githubusercontent.com/146436/27918442-5d57624e-6245-11e7-9436-5ba31cc8bd1a.png)
![screenshot from 2017-07-06 12-10-44](https://user-images.githubusercontent.com/146436/27918454-63b486da-6245-11e7-9eb5-c03aa5c40943.png)



offsetY,
        scaleX,
        scaleY,
        subtype: group.smask.subtype,
        backdrop: group.smask.backdrop,
        transferMap: group.smask.transferMap || null,
        startTransformInverse: null, 
      });
    } else {
      
      
      currentCtx.setTransform(1, 0, 0, 1, 0, 0);
      currentCtx.translate(offsetX, offsetY);
      currentCtx.scale(scaleX, scaleY);
    }
    
    
    copyCtxState(currentCtx, groupCtx);
    this.ctx = groupCtx;
    this.setGState([
      ["BM", "source-over"],
      ["ca", 1],
      ["CA", 1],
    ]);
    this.groupStack.push(currentCtx);
    this.groupLevel++;
    
    this.current.activeSMask = null;
  }
  endGroup(group) {