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 characterScaleX = width / measuredWidth;
          restoreNeeded = true;
          ctx.save();
          ctx.scale(characterScaleX, 1);
          scaledX /= characterScaleX;
        } else if (width !== measuredWidth) {
          scaledX +=
            (((width - measuredWidth) / 2000) * fontSize) / fontSizeScale;
        }
      }
      
      
      if (this.contentVisible && (glyph.isInFont || font.missingFile)) {
        if (simpleFillText && !accent) {
          
          ctx.fillText(character, scaledX, scaledY);
        } else {
          this.paintChar(
            character,
            scaledX,
            scaledY,
            patternTransform,
            resetLineWidthToOne
          );
          if (accent) {
            const scaledAccentX =
              scaledX + (fontSize * accent.offset.x) / fontSizeScale;
            const scaledAccentY =
              scaledY - (fontSize * accent.offset.y) / fontSizeScale;
            this.paintChar(