Epsilon Offsets

[trevordblack]

Lots of discussion in how to deal with intersection inaccuracies and surface acne happening in #137.

That issue is also lots of things jumbled into one issue. I carved out epsilons and put it into its own issue.

Ref. “A Fast and Robust Method for Avoiding Self-Intersection”, Ray Tracing Gems, part II, chapter 6.

[hollasch]

There's another epsilon lurking in constant_medium.h as well. Search for rec1.t.

[trevordblack]

ack "\.000"