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)

if (!this.current.activeSMask) {
      return;
    }
    if (!dirtyBox) {
      dirtyBox = [0, 0, this.ctx.canvas.width, this.ctx.canvas.height];
    } else {
      dirtyBox[0] = Math.floor(dirtyBox[0]);
      dirtyBox[1] = Math.floor(dirtyBox[1]);
      dirtyBox[2] = Math.ceil(dirtyBox[2]);
      dirtyBox[3] = Math.ceil(dirtyBox[3]);
    }
    const smask = this.current.activeSMask;
    const suspendedCtx = this.suspendedCtx;
    composeSMask(suspendedCtx, smask, this.ctx, dirtyBox);
    
    
    this.ctx.save();
    this.ctx.setTransform(1, 0, 0, 1, 0, 0);
    this.ctx.clearRect(0, 0, this.ctx.canvas.width, this.ctx.canvas.height);
    this.ctx.restore();
  }
  save() {