Auto merge of #16155 - chenpighead:gecko-glue-for-font-language-override, r=emilio

stylo: Implement gecko glue for font-language-override.

To be aligned with the implementation from Gecko side, we parse
font-language-override as Normal keyword or String. Then, we compute
and store it as a u32. So, as to the stylo glue, we can just pass the
u32 to Gecko.

The extra crate, byteorder, is used to simplify the computing and
serialization.

Since we now implement font-language-override for Gecko, we can remove
the additional branches for font-language-override in font shorthand.

ref: Gecko [Bug 1347821](https://bugzilla.mozilla.org/show_bug.cgi?id=1347821)

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Cargo.lock

@@ -2746,6 +2746,7 @@ dependencies = [
  "atomic_refcell 0.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "bindgen 0.22.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "bitflags 0.7.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "byteorder 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "cfg-if 0.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "cssparser 0.12.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "encoding 0.2.33 (registry+https://github.com/rust-lang/crates.io-index)",