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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Cargo.lock

@@ -906,6 +906,7 @@ dependencies = [
  "log 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)",
  "num_cpus 1.2.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "parking_lot 0.3.8 (registry+https://github.com/rust-lang/crates.io-index)",
+ "rayon 0.6.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "selectors 0.18.0",
  "servo_url 0.0.1",
  "style 0.0.1",
@@ -2721,7 +2722,6 @@ dependencies = [
  "nsstring_vendor 0.1.0",
  "num-integer 0.1.32 (registry+https://github.com/rust-lang/crates.io-index)",
  "num-traits 0.1.36 (registry+https://github.com/rust-lang/crates.io-index)",
- "num_cpus 1.2.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "ordered-float 0.4.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "owning_ref 0.2.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "parking_lot 0.3.8 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -2794,6 +2794,7 @@ dependencies = [
  "log 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)",
  "num_cpus 1.2.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "parking_lot 0.3.8 (registry+https://github.com/rust-lang/crates.io-index)",
+ "rayon 0.6.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "selectors 0.18.0",
  "servo_url 0.0.1",
  "style 0.0.1",