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.current.leading = -leading;
  }
  setFont(fontRefName, size) {
    const fontObj = this.commonObjs.get(fontRefName);
    const current = this.current;
    if (!fontObj) {
      throw new Error(`Can't find font for ${fontRefName}`);
    }
    current.fontMatrix = fontObj.fontMatrix || FONT_IDENTITY_MATRIX;
    
    
    if (current.fontMatrix[0] === 0 || current.fontMatrix[3] === 0) {
      warn("Invalid font matrix for font " + fontRefName);
    }
    
    
    if (size < 0) {
      size = -size;
      current.fontDirection = -1;
    } else {
      current.fontDirection = 1;
    }
    this.current.font = fontObj;
    this.current.fontSize = size;
    if (fontObj.isType3Font) {
      return; 
    }
    const name = fontObj.loadedName || "sans-serif";
    let bold = "normal";
    if (fontObj.black) {