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)

if (isNum(glyph)) {
        x += (spacingDir * glyph * fontSize) / 1000;
        continue;
      }
      let restoreNeeded = false;
      const spacing = (glyph.isSpace ? wordSpacing : 0) + charSpacing;
      const character = glyph.fontChar;
      const accent = glyph.accent;
      let scaledX, scaledY;
      let width = glyph.width;
      if (vertical) {
        const vmetric = glyph.vmetric || defaultVMetrics;
        const vx =
          -(glyph.vmetric ? vmetric[1] : width * 0.5) * widthAdvanceScale;
        const vy = vmetric[2] * widthAdvanceScale;
        width = vmetric ? -vmetric[0] : width;
        scaledX = vx / fontSizeScale;
        scaledY = (x + vy) / fontSizeScale;
      } else {
        scaledX = x / fontSizeScale;
        scaledY = 0;
      }
      if (font.remeasure && width > 0) {
        
        
        
        const measuredWidth =
          ((ctx.measureText(character).width * 1000) / fontSize) *
          fontSizeScale;
        if (width < measuredWidth && this.isFontSubpixelAAEnabled) {