This tolerance allows faces that are perpendicular to the view vector to be ignored while calculating occlusion. Files mentioned in T88675 now works very nicely.

This patch works especially in ortho cameras, also it will work in perspective cameras when the stars aligned.

I haven't find any problems that this fix has caused in my existing files, so I need more people to test it.

(I recommend this to get into master without applying other weird patches from line art branch. It's not conflicting with any of those functions.)