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)

bold = "900";
    } else if (fontObj.bold) {
      bold = "bold";
    }
    const italic = fontObj.italic ? "italic" : "normal";
    const typeface = `"${name}", ${fontObj.fallbackName}`;
    
    
    
    
    let browserFontSize = size;
    if (size < MIN_FONT_SIZE) {
      browserFontSize = MIN_FONT_SIZE;
    } else if (size > MAX_FONT_SIZE) {
      browserFontSize = MAX_FONT_SIZE;
    }
    this.current.fontSizeScale = size / browserFontSize;
    this.ctx.font = `${italic} ${bold} ${browserFontSize}px ${typeface}`;
  }
  setTextRenderingMode(mode) {
    this.current.textRenderingMode = mode;
  }
  setTextRise(rise) {
    this.current.textRise = rise;
  }
  moveText(x, y) {
    this.current.x = this.current.lineX += x;
    this.current.y = this.current.lineY += y;
  }
  setLeadingMoveText(x, y) {