Chapter 5 "Speeding it up even more - recalculating visibility when we need to" doesn't check dirty

In [this section](https://github.com/thebracket/rustrogueliketutorial/blob/master/book/src/chapter_5.md#speeding-it-up-even-more---recalculating-visibility-when-we-need-to) of Chapter 5, it sounds like the section should have a code snippet for checking whether the `dirty` flag is set, and in fact alludes to it:

> Our system can be extended to check if the dirty flag is true, and only recalculate if it is - and set the dirty flag to false when it is done.

Are we actually supposed to do this here, or in a later section..?

> The game now runs *very* fast once more

Kinda implies that we should have made the change ourselves and it should be faster now.