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.groupLevel--;
    const groupCtx = this.ctx;
    const { ctx, suspendedCtx } = this.groupStack.pop();
    this.ctx = ctx;
    
    
    this.ctx.imageSmoothingEnabled = false;
    if (suspendedCtx) {
      this.suspendedCtx = suspendedCtx;
    }
    if (group.smask) {
      this.tempSMask = this.smaskStack.pop();
      this.restore();
    } else {
      this.ctx.restore();
      const currentMtx = this.ctx.mozCurrentTransform;
      this.restore();
      this.ctx.save();
      this.ctx.setTransform.apply(this.ctx, currentMtx);
      const dirtyBox = Util.getAxialAlignedBoundingBox(
        [0, 0, groupCtx.canvas.width, groupCtx.canvas.height],
        currentMtx
      );
      this.ctx.drawImage(groupCtx.canvas, 0, 0);
      this.ctx.restore();
      this.compose(dirtyBox);
    }
  }
  beginAnnotations() {