Move RenderingContext to webrender_traits (#32386)

The `gfx_traits` crate is gradually become just about text and fonts and
this is one of the few things there that is standing in the way of this.
Eventually `webrender_traits` will be the general cross-process
compositor API, so this sort of makes sense as well.

components/gfx/lib.rs

@@ -11,5 +11,4 @@ pub mod font_store;
 pub mod font_template;
 #[allow(unsafe_code)]
 pub mod platform;
-pub mod rendering_context;
 pub mod text;