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 minY = Math.round(y1),
    maxY = Math.round(y3);
  let xa, car, cag, cab;
  let xb, cbr, cbg, cbb;
  for (let y = minY; y <= maxY; y++) {
    if (y < y2) {
      let k;
      if (y < y1) {
        k = 0;
      } else {
        k = (y1 - y) / (y1 - y2);
      }
      xa = x1 - (x1 - x2) * k;
      car = c1r - (c1r - c2r) * k;
      cag = c1g - (c1g - c2g) * k;
      cab = c1b - (c1b - c2b) * k;
    } else {
      let k;
      if (y > y3) {
        k = 1;
      } else if (y2 === y3) {
        k = 0;
      } else {
        k = (y2 - y) / (y2 - y3);
      }
      xa = x2 - (x2 - x3) * k;
      car = c2r - (c2r - c3r) * k;
      cag = c2g - (c2g - c3g) * k;
      cab = c2b - (c2b - c3b) * k;
    }