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):

let pattern;
    if (IR[0] === "TilingPattern") {
      const color = IR[1];
      const baseTransform =
        this.baseTransform || this.ctx.mozCurrentTransform.slice();
      const canvasGraphicsFactory = {
        createCanvasGraphics: ctx => {
          return new CanvasGraphics(
            ctx,
            this.commonObjs,
            this.objs,
            this.canvasFactory
          );
        },
      };
      pattern = new TilingPattern(
        IR,
        color,
        this.ctx,
        canvasGraphicsFactory,
        baseTransform
      );
    } else {
      pattern = this._getPattern(IR[1], IR[2]);
    }
    return pattern;
  }
  setStrokeColorN() {