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 height = imgData.height,
    width = imgData.width;
  const partialChunkHeight = height % FULL_CHUNK_HEIGHT;
  const fullChunks = (height - partialChunkHeight) / FULL_CHUNK_HEIGHT;
  const totalChunks = partialChunkHeight === 0 ? fullChunks : fullChunks + 1;
  const chunkImgData = ctx.createImageData(width, FULL_CHUNK_HEIGHT);
  let srcPos = 0;
  const src = imgData.data;
  const dest = chunkImgData.data;
  for (let i = 0; i < totalChunks; i++) {
    const thisChunkHeight =
      i < fullChunks ? FULL_CHUNK_HEIGHT : partialChunkHeight;
    
    
    let destPos = 3; 
    for (let j = 0; j < thisChunkHeight; j++) {
      let elem,
        mask = 0;
      for (let k = 0; k < width; k++) {
        if (!mask) {
          elem = src[srcPos++];
          mask = 128;
        }
        dest[destPos] = elem & mask ? 0 : 255;
        destPos += 4;
        mask >>= 1;