This approach gets rid of iuser->pass for good.
Also, I'm commenting out the pass increase/decrease. This was broken
since multiview. I will fix it later (before 2.75), but I didn't want to
get this patch mangled with that fix.
Thanks Sergey Sharybin for the review and feedbacks.
Reviewers: sergey
Differential Revision: https://developer.blender.org/D1232