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).
graphics.endPath();
      }
    },
    setFillAndStrokeStyleToContext: function setFillAndStrokeStyleToContext(
      graphics,
      paintType,
      color
    ) {
      const context = graphics.ctx,
        current = graphics.current;
      switch (paintType) {
        case PaintType.COLORED:
          const ctx = this.ctx;
          context.fillStyle = ctx.fillStyle;
          context.strokeStyle = ctx.strokeStyle;
          current.fillColor = ctx.fillStyle;
          current.strokeColor = ctx.strokeStyle;
          break;
        case PaintType.UNCOLORED:
          const cssColor = Util.makeHexColor(color[0], color[1], color[2]);
          context.fillStyle = cssColor;
          context.strokeStyle = cssColor;
          
          current.fillColor = cssColor;
          current.strokeColor = cssColor;
          break;
        default:
          throw new FormatError(`Unsupported paint type: ${paintType}`);
      }
    },