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)

ctx.fillText(character, x, y);
      }
      if (
        fillStrokeMode === TextRenderingMode.STROKE ||
        fillStrokeMode === TextRenderingMode.FILL_STROKE
      ) {
        if (resetLineWidthToOne) {
          ctx.save();
          ctx.moveTo(x, y);
          ctx.resetTransform();
          ctx.lineWidth = Math.round(this._combinedScaleFactor);
          ctx.strokeText(character, 0, 0);
          ctx.restore();
        } else {
          ctx.strokeText(character, x, y);
        }
      }
    }
    if (isAddToPathSet) {
      const paths = this.pendingTextPaths || (this.pendingTextPaths = []);
      paths.push({
        transform: ctx.mozCurrentTransform,
        x,
        y,
        fontSize,
        addToPath,
      });
    }
  }
  get isFontSubpixelAAEnabled() {