Reduce dependency on shared mutable state in util::opts

[djc]

As I'm familiarizing myself with the Servo code base, I came across the util::opts module. Having the option state there and having numerous crates depend on that state seems like bad design, creating a weirdly tight coupling between these crates (including compositing, gfx, layout, net and a few others) and the util::opts module.

In 29c7c5b, I already severed the dependency of the canvas crate on util::opts. With #12560, I'm trying to remove the usage of the layout_threads option from the layout_thread crate.

[nox]

The situation on that regard could still be improved.

[dralley]

Will layout 2020 address this?

[jdm]

This is unrelated to layout 2020. However, this is tracked in more detail in #22854.