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.