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/event_loop.rs

@@ -9,6 +9,8 @@
 //! handle events.
 use std::marker::PhantomData;
 use std::ops::Deref;
+#[cfg(any(x11_platform, wayland_platform))]
+use std::os::unix::io::{AsFd, AsRawFd, BorrowedFd, RawFd};
 use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
 use std::{error, fmt};
 
@@ -262,6 +264,34 @@ unsafe impl<T> rwh_05::HasRawDisplayHandle for EventLoop<T> {
     }
 }
 
+#[cfg(any(x11_platform, wayland_platform))]
+impl<T> AsFd for EventLoop<T> {
+    /// Get the underlying [EventLoop]'s `fd` which you can register
+    /// into other event loop, like [`calloop`] or [`mio`]. When doing so, the
+    /// loop must be polled with the [`pump_events`] API.
+    ///
+    /// [`calloop`]: https://crates.io/crates/calloop
+    /// [`mio`]: https://crates.io/crates/mio
+    /// [`pump_events`]: crate::platform::pump_events::EventLoopExtPumpEvents::pump_events
+    fn as_fd(&self) -> BorrowedFd<'_> {
+        self.event_loop.as_fd()
+    }
+}
+
+#[cfg(any(x11_platform, wayland_platform))]
+impl<T> AsRawFd for EventLoop<T> {
+    /// Get the underlying [EventLoop]'s raw `fd` which you can register
+    /// into other event loop, like [`calloop`] or [`mio`]. When doing so, the
+    /// loop must be polled with the [`pump_events`] API.
+    ///
+    /// [`calloop`]: https://crates.io/crates/calloop
+    /// [`mio`]: https://crates.io/crates/mio
+    /// [`pump_events`]: crate::platform::pump_events::EventLoopExtPumpEvents::pump_events
+    fn as_raw_fd(&self) -> RawFd {
+        self.event_loop.as_raw_fd()
+    }
+}
+
 impl<T> Deref for EventLoop<T> {
     type Target = EventLoopWindowTarget<T>;
     fn deref(&self) -> &EventLoopWindowTarget<T> {