feat: Add an owned display handle type

This was supposed to be rolled out with the rwh v0.6 update, but it
was left behind for some reason. I've added this type back.

Signed-off-by: John Nunley <dev@notgull.net>

src/event_loop.rs

@@ -367,6 +367,15 @@ impl EventLoopWindowTarget {
     pub fn exiting(&self) -> bool {
         self.p.exiting()
     }
+
+    /// Gets a persistent reference to the underlying platform display.
+    ///
+    /// See the [`OwnedDisplayHandle`] type for more information.
+    pub fn owned_display_handle(&self) -> OwnedDisplayHandle {
+        OwnedDisplayHandle {
+            platform: self.p.owned_display_handle(),
+        }
+    }
 }
 
 #[cfg(feature = "rwh_06")]
@@ -386,6 +395,52 @@ unsafe impl rwh_05::HasRawDisplayHandle for EventLoopWindowTarget {
     }
 }
 
+/// A proxy for the underlying display handle.
+///
+/// The purpose of this type is to provide a cheaply clonable handle to the underlying
+/// display handle. This is often used by graphics APIs to connect to the underlying APIs.
+/// It is difficult to keep a handle to the [`EventLoop`] type or the [`EventLoopWindowTarget`]
+/// type. In contrast, this type involves no lifetimes and can be persisted for as long as
+/// needed.
+///
+/// For all platforms, this is one of the following:
+///
+/// - A zero-sized type that is likely optimized out.
+/// - A reference-counted pointer to the underlying type.
+#[derive(Clone)]
+pub struct OwnedDisplayHandle {
+    #[cfg_attr(not(any(feature = "rwh_05", feature = "rwh_06")), allow(dead_code))]
+    platform: platform_impl::OwnedDisplayHandle,
+}
+
+impl fmt::Debug for OwnedDisplayHandle {
+    #[inline]
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("OwnedDisplayHandle").finish_non_exhaustive()
+    }
+}
+
+#[cfg(feature = "rwh_06")]
+impl rwh_06::HasDisplayHandle for OwnedDisplayHandle {
+    #[inline]
+    fn display_handle(&self) -> Result<rwh_06::DisplayHandle<'_>, rwh_06::HandleError> {
+        let raw = self.platform.raw_display_handle_rwh_06()?;
+
+        // SAFETY: The underlying display handle should be safe.
+        let handle = unsafe { rwh_06::DisplayHandle::borrow_raw(raw) };
+
+        Ok(handle)
+    }
+}
+
+#[cfg(feature = "rwh_05")]
+unsafe impl rwh_05::HasRawDisplayHandle for OwnedDisplayHandle {
+    #[inline]
+    fn raw_display_handle(&self) -> rwh_05::RawDisplayHandle {
+        self.platform.raw_display_handle_rwh_05()
+    }
+}
+
 /// Used to send custom events to [`EventLoop`].
 pub struct EventLoopProxy<T: 'static> {
     event_loop_proxy: platform_impl::EventLoopProxy<T>,