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)



if (hasBackdrop) {
      composeSMaskBackdrop(maskData.data, r0, g0, b0);
    }
    composeFn(maskData.data, layerData.data, transferMap);
    maskCtx.putImageData(layerData, 0, row);
  }
}
function composeSMask(ctx, smask, layerCtx) {
  const mask = smask.canvas;
  const maskCtx = smask.context;
  ctx.setTransform(
    smask.scaleX,
    0,
    0,
    smask.scaleY,
    smask.offsetX,
    smask.offsetY
  );
  genericComposeSMask(
    maskCtx,
    layerCtx,
    mask.width,
    mask.height,
    smask.subtype,
    smask.backdrop,
    smask.transferMap
  );
  ctx.drawImage(mask, 0, 0);
}
function getImageSmoothingEnabled(transform, interpolate) {