state: Store layout override

Not used for any externally observable effects

src/receiver.rs

@@ -0,0 +1,15 @@
+/*! Defines the application-wide message bus for updating state.*/
+
+use crate::event_loop::driver::Threaded;
+
+pub mod c {
+    use super::*;
+    use crate::util::c::Wrapped;
+    pub type State = Wrapped<Threaded>;
+}
+
+// The state receiver is an endpoint of a channel, so it's safely cloneable.
+// There's no need to keep it in a Rc.
+// The C version uses Wrapped with an underlying Rc,
+// because Wrapped is well-tested already.
+pub type State = Threaded;