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 { context: ctx } = this.cachedCanvases.getCanvas(
      "isFontSubpixelAAEnabled",
      10,
      10
    );
    ctx.scale(1.5, 1);
    ctx.fillText("I", 0, 10);
    const data = ctx.getImageData(0, 0, 10, 10).data;
    let enabled = false;
    for (let i = 3; i < data.length; i += 4) {
      if (data[i] > 0 && data[i] < 255) {
        enabled = true;
        break;
      }
    }
    return shadow(this, "isFontSubpixelAAEnabled", enabled);
  }
  showText(glyphs) {