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 color = Util.makeHexColor(r, g, b);
    this.ctx.strokeStyle = color;
    this.current.strokeColor = color;
  }
  setFillRGBColor(r, g, b) {
    const color = Util.makeHexColor(r, g, b);
    this.ctx.fillStyle = color;
    this.current.fillColor = color;
    this.current.patternFill = false;
  }
  _getPattern(objId, matrix = null) {
    let pattern;
    if (this.cachedPatterns.has(objId)) {
      pattern = this.cachedPatterns.get(objId);
    } else {
      pattern = getShadingPattern(
        this.objs.get(objId),
        this.cachedCanvasPatterns
      );
      this.cachedPatterns.set(objId, pattern);
    }
    if (matrix) {
      pattern.matrix = matrix;
    }
    return pattern;
  }
  shadingFill(objId) {