wlroots: Fix crash when socket is overwhelmed

Previously when the output buffer was overwhelmed, additional
events were submitted until the outgoing buffer filled up, which
causes the wayland-connection to 'break' and not accept further attempts
to flush() the socket.

src/backend/consumer/x11.rs

@@ -31,7 +31,7 @@ impl X11Consumer {
 }
 
 impl EventConsumer for X11Consumer {
-    fn consume(&self, event: Event, _: ClientHandle) {
+    fn consume(&mut self, event: Event, _: ClientHandle) {
         match event {
             Event::Pointer(pointer_event) => match pointer_event {
                 crate::event::PointerEvent::Motion {