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).
tmpCtx.clearRect(0, 0, tmpCtx.canvas.width, tmpCtx.canvas.height);
        tmpCtx.beginPath();
        tmpCtx.rect(0, 0, tmpCtx.canvas.width, tmpCtx.canvas.height);
        if (!shadingFill) {
          tmpCtx.setTransform.apply(tmpCtx, owner.baseTransform);
          if (matrix) {
            tmpCtx.transform.apply(tmpCtx, matrix);
          }
        } else {
          tmpCtx.setTransform.apply(tmpCtx, ctx.mozCurrentTransform);
        }
        applyBoundingBox(tmpCtx, bbox);
        let grad;
        if (type === "axial") {
          grad = tmpCtx.createLinearGradient(p0[0], p0[1], p1[0], p1[1]);
        } else if (type === "radial") {
          grad = tmpCtx.createRadialGradient(
            p0[0],
            p0[1],
            r0,
            p1[0],
            p1[1],
            r1
          );
        }
        for (let i = 0, ii = colorStops.length; i < ii; ++i) {
          const c = colorStops[i];
          grad.addColorStop(c[0], c[1]);
        }
        tmpCtx.fillStyle = grad;