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)

let k;
    if (y < y1) {
      k = 0;
    } else if (y > y3) {
      k = 1;
    } else {
      k = (y1 - y) / (y1 - y3);
    }
    xb = x1 - (x1 - x3) * k;
    cbr = c1r - (c1r - c3r) * k;
    cbg = c1g - (c1g - c3g) * k;
    cbb = c1b - (c1b - c3b) * k;
    const x1_ = Math.round(Math.min(xa, xb));
    const x2_ = Math.round(Math.max(xa, xb));
    let j = rowSize * y + x1_ * 4;
    for (let x = x1_; x <= x2_; x++) {
      k = (xa - x) / (xa - xb);
      if (k < 0) {
        k = 0;
      } else if (k > 1) {
        k = 1;
      }
      bytes[j++] = (car - (car - cbr) * k) | 0;
      bytes[j++] = (cag - (cag - cbg) * k) | 0;
      bytes[j++] = (cab - (cab - cbb) * k) | 0;
      bytes[j++] = 255;
    }
  }
}
function drawFigure(data, figure, context) {