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):

tmpCtx.rect(0, 0, tmpCtx.canvas.width, tmpCtx.canvas.height);
        tmpCtx.setTransform.apply(tmpCtx, owner.baseTransform);
        if (this.matrix) {
          tmpCtx.transform.apply(tmpCtx, this.matrix);
        }
        applyBoundingBox(tmpCtx, this._bbox);
        tmpCtx.fillStyle = this._createGradient(tmpCtx);
        tmpCtx.fill();
        pattern = ctx.createPattern(tmpCanvas.canvas, "repeat");
        this.cachedCanvasPatterns.set(this, pattern);
      }
    } else {
      
      applyBoundingBox(ctx, this._bbox);
      pattern = this._createGradient(ctx);
    }
    if (!shadingFill) {
      const domMatrix = new DOMMatrix(inverse);
      try {
        pattern.setTransform(domMatrix);
      } catch (ex) {
        
        
        warn(`RadialAxialShadingPattern.getPattern: "${ex?.message}".`);
      }
    }
    return pattern;
  }
}
function drawTriangle(data, context, p1, p2, p3, c1, c2, c3) {