Do not use ParallelIterator if not using rayon
The rayon [`ParalllelIIterator`](https://github.com/rayon-rs/rayon/blob/master/src/iter/plumbing/README.md) is being used for inline content size calculations irrespective of the `use_rayon` flag, which can lead to an unbounded (dynamically determined) number of threads being spawned during iteration.
The check is performed for static position calculations, however:
6fced22e47/components/layout_2020/flow/mod.rs (L235-L242)
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