No more headless compositor. Just the normal one.

This changes headless operation to strictly be a runtime option, rather
than a compile-time one. Note that the old headless version still relied
on a display server to support WebGL, while it now requires one all the
time.

Fixes #8573

components/script/script_thread.rs

@@ -552,6 +552,7 @@ impl ScriptThreadFactory for ScriptThread {
             let reporter_name = format!("script-reporter-{}", id);
             mem_profiler_chan.run_with_memory_reporting(|| {
                 script_thread.start();
+                let _ = script_thread.compositor.borrow_mut().send(ScriptToCompositorMsg::Exited);
                 let _ = script_thread.content_process_shutdown_chan.send(());
             }, reporter_name, channel_for_reporter, CommonScriptMsg::CollectReports);