The Ray Tracer Challenge: hit inconsistency

@jamis

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Title: The Ray Tracer Challenge

There seems to be some inconsistency about hits. In some parts of the book, the hit is described as the intersection across all shapes in the world with the lowest non-negative t value, whereas in one part of the book, the hit is described as the intersection across all shapes in the world with the lowest positive t value.

I implemented the code to find the intersection with the lowest non-negative t value, which, in chapter 8, when adding in_shadow, caused a lot of my formerly passing world tests to suddenly fail. I was extremely confused and spent a lot of time trying to find out what was happening.

I looked at an old implementation of the ray tracer I had done which technically reviewing this book, and the hit I had defined had the lowest positive t value. Sure enough, when I changed this to be the definition of hit despite the multiple references of a non-negative value, everything passed and seems to be working correctly.