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)

this._transformMatrix = old.slice(0, 6);
    this._originalSave();
  };
  ctx.restore = function ctxRestore() {
    const prev = this._transformStack.pop();
    if (prev) {
      this._transformMatrix = prev;
      this._originalRestore();
    }
  };
  ctx.translate = function ctxTranslate(x, y) {
    const m = this._transformMatrix;
    m[4] = m[0] * x + m[2] * y + m[4];
    m[5] = m[1] * x + m[3] * y + m[5];
    this._originalTranslate(x, y);
  };
  ctx.scale = function ctxScale(x, y) {
    const m = this._transformMatrix;
    m[0] *= x;
    m[1] *= x;
    m[2] *= y;
    m[3] *= y;
    this._originalScale(x, y);
  };
  ctx.transform = function ctxTransform(a, b, c, d, e, f) {
    const m = this._transformMatrix;
    this._transformMatrix = [
      m[0] * a + m[2] * b,
      m[1] * a + m[3] * b,
      m[0] * c + m[2] * d,