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 length = maskData.length;
  for (let i = 3; i < length; i += 4) {
    const y =
      maskData[i - 3] * 77 + 
      maskData[i - 2] * 152 + 
      maskData[i - 1] * 28; 
    layerData[i] = transferMap
      ? (layerData[i] * transferMap[y >> 8]) >> 8
      : (layerData[i] * y) >> 16;
  }
}
function genericComposeSMask(
  maskCtx,
  layerCtx,
  width,
  height,
  subtype,
  backdrop,
  transferMap,
  layerOffsetX,
  layerOffsetY,
  maskOffsetX,
  maskOffsetY
) {
  const hasBackdrop = !!backdrop;
  const r0 = hasBackdrop ? backdrop[0] : 0;
  const g0 = hasBackdrop ? backdrop[1] : 0;
  const b0 = hasBackdrop ? backdrop[2] : 0;
  let composeFn;