Desktop: Use multithreaded CEF event loop on Windows and Linux (#3076)

* Prototype multi threaded event loop

* Fix input event dispatch

* Remove dead code

* Reenable do_message_loop_work for macos targets

* Cleanup

* Review cleanup

* Remove outdated comment

* Attempt to fix texture import errors

---------

Co-authored-by: Timon Schelling <me@timon.zip>

desktop/src/cef/context/multithreaded.rs

@@ -0,0 +1,67 @@
+use cef::sys::cef_thread_id_t;
+use cef::{Task, ThreadId, post_task};
+use std::cell::RefCell;
+use winit::event::WindowEvent;
+
+use crate::cef::internal::task::ClosureTask;
+
+use super::CefContext;
+use super::singlethreaded::SingleThreadedCefContext;
+
+thread_local! {
+	pub(super) static CONTEXT: RefCell<Option<SingleThreadedCefContext>> = const { RefCell::new(None) };
+}
+
+pub(super) struct MultiThreadedCefContextProxy;
+
+impl CefContext for MultiThreadedCefContextProxy {
+	fn work(&mut self) {
+		// CEF handles its own message loop in multi-threaded mode
+	}
+
+	fn handle_window_event(&mut self, event: &WindowEvent) {
+		let event_clone = event.clone();
+		run_on_ui_thread(move || {
+			CONTEXT.with(|b| {
+				if let Some(context) = b.borrow_mut().as_mut() {
+					context.handle_window_event(&event_clone);
+				}
+			});
+		});
+	}
+
+	fn notify_of_resize(&self) {
+		run_on_ui_thread(move || {
+			CONTEXT.with(|b| {
+				if let Some(context) = b.borrow_mut().as_mut() {
+					context.notify_of_resize();
+				}
+			});
+		});
+	}
+
+	fn send_web_message(&self, message: Vec<u8>) {
+		run_on_ui_thread(move || {
+			CONTEXT.with(|b| {
+				if let Some(context) = b.borrow_mut().as_mut() {
+					context.send_web_message(message);
+				}
+			});
+		});
+	}
+}
+
+impl Drop for MultiThreadedCefContextProxy {
+	fn drop(&mut self) {
+		cef::shutdown();
+	}
+}
+
+pub(super) fn run_on_ui_thread<F>(closure: F)
+where
+	F: FnOnce() + Send + 'static,
+{
+	let closure_task = ClosureTask::new(closure);
+	let mut task = Task::new(closure_task);
+	post_task(ThreadId::from(cef_thread_id_t::TID_UI), Some(&mut task));
+}