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)

backdrop: group.smask.backdrop,
        transferMap: group.smask.transferMap || null,
        startTransformInverse: null, 
      });
    } else {
      
      
      currentCtx.setTransform(1, 0, 0, 1, 0, 0);
      currentCtx.translate(offsetX, offsetY);
      currentCtx.scale(scaleX, scaleY);
      currentCtx.save();
    }
    
    
    copyCtxState(currentCtx, groupCtx);
    this.ctx = groupCtx;
    this.setGState([
      ["BM", "source-over"],
      ["ca", 1],
      ["CA", 1],
    ]);
    this.groupStack.push({
      ctx: currentCtx,
      suspendedCtx,
    });
    this.groupLevel++;
  }
  endGroup(group) {
    if (!this.contentVisible) {
      return;