gl: Provide raw handle() getter function on GLDisplay

Having access to the raw handle is useful when sharing the display
and context with an existing application.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1344>

gstreamer-gl/Gir.toml

@@ -346,7 +346,7 @@ status = "generate"
     [[object.function]]
     name = "get_handle"
     # return handle
-    ignore = true
+    manual = true
 
     [[object.function]]
     name = "get_gl_context_for_thread"

gstreamer-gl/src/gl_display.rs

@@ -9,6 +9,12 @@ mod sealed {
 }
 
 pub trait GLDisplayExtManual: sealed::Sealed + IsA<GLDisplay> + 'static {
+    #[doc(alias = "gst_gl_display_get_handle")]
+    #[doc(alias = "get_handle")]
+    fn handle(&self) -> usize {
+        unsafe { ffi::gst_gl_display_get_handle(self.as_ref().to_glib_none().0) }
+    }
+
     #[cfg(feature = "v1_24")]
     #[cfg_attr(docsrs, doc(cfg(feature = "v1_24")))]
     #[doc(alias = "gst_gl_display_ensure_context")]