Rewrite book 1 diffuse in more straightforward way

[trevordblack]

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In think that when implementing recursive bouncing for the first time, sampling a direction uniformly is one of the first things to try.

This is actually how I would rewrite the chapter. Start with the "naive" approach, and then correct toward cosine law. We start on cos^2 today (I literally cannot remember if it's cos^3 or cos^2) because that was the outcome of the algorithm in the initial version of the book, and so we say upfront that that way of doing it was wrong. With enough time past since the initial publication of v1, it might be worth rearranging them.

Originally posted by @trevordblack in #924 (comment)