The webrender_api crate contains an assortment types and functions used
by WebRender consumers as well as, in many cases, WebRender itself.
This separation allows Servo to parallelize compilation across webrender
and other crates that depend on webrender_api. So in practice, we put
things in this crate when Servo needs to use them. Firefox depends on the
webrender crate directly, and so this distinction is not really relevant
there.