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):

ctx.__originalTransform = ctx.transform;
  ctx.__originalSetTransform = ctx.setTransform;
  ctx.__originalResetTransform = ctx.resetTransform;
  ctx.__originalClip = ctx.clip;
  ctx.__originalMoveTo = ctx.moveTo;
  ctx.__originalLineTo = ctx.lineTo;
  ctx.__originalBezierCurveTo = ctx.bezierCurveTo;
  ctx.__originalRect = ctx.rect;
  ctx.__originalClosePath = ctx.closePath;
  ctx.__originalBeginPath = ctx.beginPath;
  ctx._removeMirroring = () => {
    ctx.save = ctx.__originalSave;
    ctx.restore = ctx.__originalRestore;
    ctx.rotate = ctx.__originalRotate;
    ctx.scale = ctx.__originalScale;
    ctx.translate = ctx.__originalTranslate;
    ctx.transform = ctx.__originalTransform;
    ctx.setTransform = ctx.__originalSetTransform;
    ctx.resetTransform = ctx.__originalResetTransform;
    ctx.clip = ctx.__originalClip;
    ctx.moveTo = ctx.__originalMoveTo;
    ctx.lineTo = ctx.__originalLineTo;
    ctx.bezierCurveTo = ctx.__originalBezierCurveTo;
    ctx.rect = ctx.__originalRect;
    ctx.closePath = ctx.__originalClosePath;
    ctx.beginPath = ctx.__originalBeginPath;
    delete ctx._removeMirroring;
  };
  ctx.save = function ctxSave() {
    destCtx.save();