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 ps = figure.coords;
    const cs = figure.colors;
    let i, ii;
    switch (figure.type) {
      case "lattice":
        const verticesPerRow = figure.verticesPerRow;
        const rows = Math.floor(ps.length / verticesPerRow) - 1;
        const cols = verticesPerRow - 1;
        for (i = 0; i < rows; i++) {
          let q = i * verticesPerRow;
          for (let j = 0; j < cols; j++, q++) {
            drawTriangle(
              data,
              context,
              ps[q],
              ps[q + 1],
              ps[q + verticesPerRow],
              cs[q],
              cs[q + 1],
              cs[q + verticesPerRow]
            );
            drawTriangle(
              data,
              context,