* clippy: fix warnings in components/gfx
* refactor: switched the order of impl so that its intent is clearer
* fix: add font context default in other platforms
This is a small cleanup that moves and renames this class. The rename is
simply because we are exposing a lot about the details of Servo's
rendering in the API and it makes sense to start thinking about
abstracting that away a bit.
This also moves the struct to `gfx`, which does have an effect on
Servo's dependency graph. This adds a new dependency on gfx to
`compositing`, but `compositing` had a transitive dependency on
gfx before through `canvas`.
