Fix race condition when destroying proxies
We have a race condition (#86) when destroying proxies, because we call wl_proxy_marshal() wl_proxy_destroy()
Both of which hold and release the display mutex. It's possible for another thread to be scheduled when the mutex is dropped, and bad things happen.
Fix this by introducing a new wl_proxy_marshal_flags() call that allows us to atomically marshal and destroy under one lock.