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):

this.setLeading(-y);
    this.moveText(x, y);
  }
  setTextMatrix(a, b, c, d, e, f) {
    this.current.textMatrix = [a, b, c, d, e, f];
    this.current.textMatrixScale = Math.hypot(a, b);
    this.current.x = this.current.lineX = 0;
    this.current.y = this.current.lineY = 0;
  }
  nextLine() {
    this.moveText(0, this.current.leading);
  }
  paintChar(character, x, y, patternTransform, resetLineWidthToOne) {
    const ctx = this.ctx;
    const current = this.current;
    const font = current.font;
    const textRenderingMode = current.textRenderingMode;
    const fontSize = current.fontSize / current.fontSizeScale;
    const fillStrokeMode =
      textRenderingMode & TextRenderingMode.FILL_STROKE_MASK;
    const isAddToPathSet = !!(
      textRenderingMode & TextRenderingMode.ADD_TO_PATH_FLAG
    );
    const patternFill = current.patternFill && !font.missingFile;
    let addToPath;
    if (font.disableFontFace || isAddToPathSet || patternFill) {
      addToPath = font.getPathGenerator(this.commonObjs, character);
    }