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.

components/script/script_thread.rs

@@ -1143,7 +1143,6 @@ impl ScriptThread {
             new_pipeline_id,
             frame_type,
             load_data,
-            paint_chan,
             pipeline_port,
             layout_to_constellation_chan,
             content_process_shutdown_chan,
@@ -1160,7 +1159,6 @@ impl ScriptThread {
             layout_pair: layout_pair,
             pipeline_port: pipeline_port,
             constellation_chan: layout_to_constellation_chan,
-            paint_chan: paint_chan,
             script_chan: self.control_chan.clone(),
             image_cache_thread: self.image_cache_thread.clone(),
             content_process_shutdown_chan: content_process_shutdown_chan,