Auto merge of #15535 - froydnj:global-style-thread-pool, r=bholley

geckolib: use a global thread pool for styling

By having a single thread pool, rather than one per document, we use less memory.  This addresses https://bugzilla.mozilla.org/show_bug.cgi?id=1324250.

This may be obvious to an experienced Rust programmer, but I went with raw pointers because trying to use `Option` global variables resulted in complaints about turning on feature flags in nightly Rust.  Since this is for stylo, nightly features are not appropriate here.

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