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)

drawnHeight = MAX_GROUP_SIZE;
    }
    this.current.startNewPathAndClipBox([0, 0, drawnWidth, drawnHeight]);
    let cacheId = "groupAt" + this.groupLevel;
    if (group.smask) {
      
      cacheId += "_smask_" + (this.smaskCounter++ % 2);
    }
    const scratchCanvas = this.cachedCanvases.getCanvas(
      cacheId,
      drawnWidth,
      drawnHeight,
      true
    );
    const groupCtx = scratchCanvas.context;
    
    
    groupCtx.scale(1 / scaleX, 1 / scaleY);
    groupCtx.translate(-offsetX, -offsetY);
    groupCtx.transform.apply(groupCtx, currentTransform);
    if (group.smask) {
      
      this.smaskStack.push({
        canvas: scratchCanvas.canvas,
        context: groupCtx,
        offsetX,
        offsetY,
        scaleX,
        scaleY,
        subtype: group.smask.subtype,