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 length = maskData.length;
  for (let i = 3; i < length; i += 4) {
    const y =
      maskData[i - 3] * 77 + 
      maskData[i - 2] * 152 + 
      maskData[i - 1] * 28; 
    layerData[i] = transferMap
      ? (layerData[i] * transferMap[y >> 8]) >> 8
      : (layerData[i] * y) >> 16;
  }
}
function genericComposeSMask(
  maskCtx,
  layerCtx,
  width,
  height,
  subtype,
  backdrop,
  transferMap,
  layerOffsetX,
  layerOffsetY,
  maskOffsetX,
  maskOffsetY
) {
  const hasBackdrop = !!backdrop;
  const r0 = hasBackdrop ? backdrop[0] : 0;
  const g0 = hasBackdrop ? backdrop[1] : 0;
  const b0 = hasBackdrop ? backdrop[2] : 0;
  let composeFn;