servo: Merge #13711 - Remove old rendering backend (from glennw:remove-old-renderer); r=larsbergstrom

This removes paint threads, rust-layers dependency, and changes
optional webrender types to be required.

The use_webrender option has been removed, however I've left
the "-w" command line option in place for now so that wpt
runner can continue to pass that. Once it's removed from there
we can also remove the -w option.

Once this stage is complete, it should be fine to change the
display list building code to generate webrender display
lists directly and avoid the conversion step.

Source-Repo: https://github.com/servo/servo
Source-Revision: f96718d03da969510dc992699cb6f25c2e21ae1e

servo/components/script_layout_interface/lib.rs

@@ -39,7 +39,6 @@ extern crate selectors;
 extern crate string_cache;
 extern crate style;
 extern crate url;
-extern crate util;
 
 pub mod message;
 pub mod reporter;