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 = bytes.length;
  for (let i = 3; i < length; i += 4) {
    const alpha = bytes[i];
    if (alpha === 0) {
      bytes[i - 3] = r0;
      bytes[i - 2] = g0;
      bytes[i - 1] = b0;
    } else if (alpha < 255) {
      const alpha_ = 255 - alpha;
      bytes[i - 3] = (bytes[i - 3] * alpha + r0 * alpha_) >> 8;
      bytes[i - 2] = (bytes[i - 2] * alpha + g0 * alpha_) >> 8;
      bytes[i - 1] = (bytes[i - 1] * alpha + b0 * alpha_) >> 8;
    }
  }
}
function composeSMaskAlpha(maskData, layerData, transferMap) {
  const length = maskData.length;
  const scale = 1 / 255;
  for (let i = 3; i < length; i += 4) {
    const alpha = transferMap ? transferMap[maskData[i]] : maskData[i];
    layerData[i] = (layerData[i] * alpha * scale) | 0;
  }
}
function composeSMaskLuminosity(maskData, layerData, transferMap) {