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)

const inSMaskMode = !!this.suspendedCtx;
    if (this.current.activeSMask && !inSMaskMode) {
      this.beginSMaskMode();
    } else if (!this.current.activeSMask && inSMaskMode) {
      this.endSMaskMode();
    }
    
  }
  /**
   * Soft mask mode takes the current main drawing canvas and replaces it with
   * a temporary canvas. Any drawing operations that happen on the temporary
   * canvas need to be composed with the main canvas that was suspended (see
   * `compose()`). The temporary canvas also duplicates many of its operations
   * on the suspended canvas to keep them in sync, so that when the soft mask
   * mode ends any clipping paths or transformations will still be active and in
   * the right order on the canvas' graphics state stack.
   */
  beginSMaskMode() {