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 (subtype === "Luminosity") {
    composeFn = composeSMaskLuminosity;
  } else {
    composeFn = composeSMaskAlpha;
  }
  
  const PIXELS_TO_PROCESS = 1048576;
  const chunkSize = Math.min(height, Math.ceil(PIXELS_TO_PROCESS / width));
  for (let row = 0; row < height; row += chunkSize) {
    const chunkHeight = Math.min(chunkSize, height - row);
    const maskData = maskCtx.getImageData(
      layerOffsetX - maskOffsetX,
      row + (layerOffsetY - maskOffsetY),
      width,
      chunkHeight
    );
    const layerData = layerCtx.getImageData(
      layerOffsetX,
      row + layerOffsetY,
      width,
      chunkHeight
    );
    if (hasBackdrop) {
      composeSMaskBackdrop(maskData.data, r0, g0, b0);
    }
    composeFn(maskData.data, layerData.data, transferMap);
    layerCtx.putImageData(layerData, layerOffsetX, row + layerOffsetY);
  }
}
function composeSMask(ctx, smask, layerCtx, layerBox) {