Link to PDF file (or attach file here):

[gs-bugzilla689309-reduced-bc1_RGB.pdf](https://github.com/mozilla/pdf.js/files/649453/gs-bugzilla689309-reduced-bc1_RGB.pdf)
[gs-bugzilla689309-reduced-bc0_RGB.pdf](https://github.com/mozilla/pdf.js/files/649454/gs-bugzilla689309-reduced-bc0_RGB.pdf)

Configuration:
- Web browser and its version: ff 50.0.2
- Operating system and its version: W7 64bit fully patched
- PDF.js version: most recent
- Is an extension: ?

Steps to reproduce the problem:
1. open demo viewer
2. open file gs-bugzilla689309-reduced-bc1_RGB.pdf

What is the expected behavior? (add screenshot)
![screenshot-adobe](https://cloud.githubusercontent.com/assets/6665575/21148639/2fb5d466-c159-11e6-9410-7dbfb5eff11f.jpg)

What went wrong? (add screenshot)
![screenshot-pdfjs](https://cloud.githubusercontent.com/assets/6665575/21148650/35e90808-c159-11e6-95cb-a60a91bff6e6.jpg)

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

Additional comment:
The two files are identical except for the /bc value in the softmask. The softmask is smaller than the transparency group it is applied to. My understanding is that the /bc color is to be used instead of the softmask as an "influence" for the area that isn't covered by the softmask. I suspect that PDF.js just ignores /bc.

The fine black line doesn't belong there (although it happens to be useful as it shows the size of the softmask). The red line does belong there and shows the size of the transparency group.

The file is a reduced and modified version of one in https://bugs.ghostscript.com/show_bug.cgi?id=688601 to investigate a different problem than the one mentioned there.
setDash: 6,
  setRenderingIntent: 7,
  setFlatness: 8,
  setGState: 9,
  save: 10,
  restore: 11,
  transform: 12,
  moveTo: 13,
  lineTo: 14,
  curveTo: 15,
  curveTo2: 16,
  curveTo3: 17,
  closePath: 18,
  rectangle: 19,
  stroke: 20,
  closeStroke: 21,
  fill: 22,
  eoFill: 23,
  fillStroke: 24,
  eoFillStroke: 25,
  closeFillStroke: 26,
  closeEOFillStroke: 27,
  endPath: 28,
  clip: 29,
  eoClip: 30,
  beginText: 31,
  endText: 32,
  setCharSpacing: 33,
  setWordSpacing: 34,
  setHScale: 35,