Link to PDF file (or attach file here):
https://issues.apache.org/jira/secure/attachment/12650283/000004_ShadingPatternTextPDF.pdf

Configuration:
- Web browser and its version: FF 51.0.1 32bit
- Operating system and its version: W7
- PDF.js version: latest

Steps to reproduce the problem:
display file

What is the expected behavior? (add screenshot)
![ok](https://cloud.githubusercontent.com/assets/6665575/23339534/5ff2de9c-fc25-11e6-837e-0fdaf81b4ca9.png)

What went wrong? (add screenshot)
![nok](https://cloud.githubusercontent.com/assets/6665575/23339536/63f7a37e-fc25-11e6-813c-cc218cc4def3.png)

possibly similar problem:
https://www.pdflib.com/fileadmin/pdflib/Cookbook/pdf/color_gradient.pdf (shaded text completely missing)
https://issues.apache.org/jira/secure/attachment/12630566/pslib-shading.pdf (page 4, part of text missing)
https://issues.apache.org/jira/secure/attachment/12652009/pattern-shading-2-4-idMatrix.pdf (some text missing)
https://issues.apache.org/jira/secure/attachment/12652010/pattern-shading-2-4-noMatrix.pdf (some text missing)

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,