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).
if (!bbox || typeof Path2D === "undefined") {
    return;
  }
  const width = bbox[2] - bbox[0];
  const height = bbox[3] - bbox[1];
  const region = new Path2D();
  region.rect(bbox[0], bbox[1], width, height);
  ctx.clip(region);
}
ShadingIRs.RadialAxial = {
  fromIR: function RadialAxial_fromIR(raw) {
    const type = raw[1];
    const bbox = raw[2];
    const colorStops = raw[3];
    const p0 = raw[4];
    const p1 = raw[5];
    const r0 = raw[6];
    const r1 = raw[7];
    const matrix = raw[8];
    return {
      getPattern: function RadialAxial_getPattern(ctx, owner, shadingFill) {
        const tmpCanvas = owner.cachedCanvases.getCanvas(
          "pattern",
          ctx.canvas.width,
          ctx.canvas.height,
          true
        );
        const tmpCtx = tmpCanvas.context;