Implement AsFd/AsRawFd for EventLoop<T>

This should help other crates to integrate winit's event loop into
their bigger event loop without adding an extra thread.

src/platform_impl/linux/wayland/event_loop/mod.rs

@@ -4,6 +4,7 @@ use std::cell::{Cell, RefCell};
 use std::io::Result as IOResult;
 use std::marker::PhantomData;
 use std::mem;
+use std::os::unix::io::{AsFd, AsRawFd, BorrowedFd, RawFd};
 use std::rc::Rc;
 use std::sync::atomic::Ordering;
 use std::sync::{Arc, Mutex};
@@ -589,6 +590,18 @@ impl<T: 'static> EventLoop<T> {
     }
 }
 
+impl<T> AsFd for EventLoop<T> {
+    fn as_fd(&self) -> BorrowedFd<'_> {
+        self.event_loop.as_fd()
+    }
+}
+
+impl<T> AsRawFd for EventLoop<T> {
+    fn as_raw_fd(&self) -> RawFd {
+        self.event_loop.as_raw_fd()
+    }
+}
+
 pub struct EventLoopWindowTarget<T> {
     /// The event loop wakeup source.
     pub event_loop_awakener: calloop::ping::Ping,