**Link to PDF file (or attach file here):**
[tocheckFile.pdf](https://github.com/mozilla/pdf.js/files/240714/tocheckFile.pdf)

**Configuration:**
- Web browser and its version: FF45.0.2, Chrome Version 50.0.2661.86 (64-bit) 
- Operating system and its version: Mac OSX El Capitan

**\- PDF.js version:**
PDFJS.version = '1.1.215';
PDFJS.build = 'c9a7498';

**Steps to reproduce the problem:**
1. Use the lib to generate a PDF preview.

**What is the expected behavior? (add screenshot)**
PDF preview should have rendered completely. (frog head should be complete)

**What went wrong? (add screenshot)**
<img width="952" alt="imagesnotrenderinginpreview" src="https://cloud.githubusercontent.com/assets/10231850/14889265/cecb2d64-0d66-11e6-98e1-da2d39728552.png">

See attached screenshot in regards to the pdf file

Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension):

maskCtx,
  layerCtx,
  width,
  height,
  subtype,
  backdrop,
  transferMap
) {
  const hasBackdrop = !!backdrop;
  const r0 = hasBackdrop ? backdrop[0] : 0;
  const g0 = hasBackdrop ? backdrop[1] : 0;
  const b0 = hasBackdrop ? backdrop[2] : 0;
  let composeFn;
  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(0, row, width, chunkHeight);
    const layerData = layerCtx.getImageData(0, row, width, chunkHeight);