Sometimes clippy gets outdated by months, and its current support setup
means that each Servo component need to opt into it by depending on
the plugins crate manually, and not all components do that.
Rename ScriptChan in constellation to EventLoop
<!-- Please describe your changes on the following line: -->
We currently have a type `ScriptChan` in the constellation, which is named after its implementation rather than its semantics. In the spec, the nearest concept seems to be event loop https://html.spec.whatwg.org/multipage/#event-loop.
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes do not require tests because renaming.
<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/14260)
<!-- Reviewable:end -->
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 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.
