egl: Fix egl display handle visibility

Currently there is no way to get the raw handle of an `EGLDisplay`, but
this can be necessary for implementing additional egl platforms out-of-tree.

src/backend/egl/display.rs

@@ -403,6 +403,11 @@ impl EGLDisplay {
         Ok((desc, config_id))
     }
 
+    /// Get a handle to the underlying native EGLDisplay
+    pub fn get_display_handle(&self) -> Arc<EGLDisplayHandle> {
+        self.display.clone()
+    }
+
     /// Returns the runtime egl version of this display
     pub fn get_egl_version(&self) -> (i32, i32) {
         self.egl_version