Link to PDF file (or attach file here):
[gradient invisible pdfjs.pdf](https://github.com/mozilla/pdf.js/files/1096729/gradient.invisible.pdfjs.pdf)

Configuration:
- Web browser and its version: 59.0.3071.109
- Operating system and its version: Windows 10
- PDF.js version: 1.8.480
- Is an extension: No

Steps to reproduce the problem:
1. Open the file in Adobe Reader or Google Chrome
2. Open the file in PDFjs online viewer
3. Compare

What is the expected behavior? (add screenshot)
- There is an orange radial gradient, from opacity 0 at the center to opacity 1 at the page bound

![image](https://user-images.githubusercontent.com/17966696/27466423-3923639e-57e3-11e7-841f-7f498732bb20.png)

What went wrong? (add screenshot)
- The gradient is completely missing

![image](https://user-images.githubusercontent.com/17966696/27466446-63e36606-57e3-11e7-86c3-9a5e0b6b0e0a.png)

Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension):
http://mozilla.github.io/pdf.js/web/viewer.html

Description of the file:
An opaque orange gradient is masked with a mask. The mask's group contains a rectangle filled with a pattern. The pattern contains an other rectangle filled with a greyscale gradient from black to white. All the rectangles, patterns & groups have the same size as the page (400x400pt).
];
      
      
      
      const dimx = this.getSizeAndScale(
        xstep,
        this.ctx.canvas.width,
        combinedScale[0]
      );
      const dimy = this.getSizeAndScale(
        ystep,
        this.ctx.canvas.height,
        combinedScale[1]
      );
      const tmpCanvas = owner.cachedCanvases.getCanvas(
        "pattern",
        dimx.size,
        dimy.size,
        true
      );
      const tmpCtx = tmpCanvas.context;
      const graphics = canvasGraphicsFactory.createCanvasGraphics(tmpCtx);
      graphics.groupLevel = owner.groupLevel;
      this.setFillAndStrokeStyleToContext(graphics, paintType, color);
      graphics.transform(dimx.scale, 0, 0, dimy.scale, 0, 0);
      this.clipBbox(graphics, bbox, x0, y0, x1, y1);
      graphics.baseTransform = graphics.ctx.mozCurrentTransform.slice();
      graphics.executeOperatorList(operatorList);
      return {
        canvas: tmpCanvas.canvas,