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):

const layerOffsetX = layerBox[0];
  const layerOffsetY = layerBox[1];
  const layerWidth = layerBox[2] - layerOffsetX;
  const layerHeight = layerBox[3] - layerOffsetY;
  if (layerWidth === 0 || layerHeight === 0) {
    return;
  }
  genericComposeSMask(
    smask.context,
    layerCtx,
    layerWidth,
    layerHeight,
    smask.subtype,
    smask.backdrop,
    smask.transferMap,
    layerOffsetX,
    layerOffsetY,
    smask.offsetX,
    smask.offsetY
  );
  ctx.save();
  ctx.globalAlpha = 1;
  ctx.globalCompositeOperation = "source-over";
  ctx.setTransform(1, 0, 0, 1, 0, 0);
  ctx.drawImage(layerCtx.canvas, 0, 0);
  ctx.restore();
}
function getImageSmoothingEnabled(transform, interpolate) {