Remove old rendering backend.

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.

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