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
89fec93361