meta: Add is_registered function for CustomMeta

Test if a Meta type is already registered.
It is useful to know if a custom metadata type has already
been registed to avoid registererd twice and get asserts.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1391>

gstreamer/src/meta.rs

@@ -951,6 +951,11 @@ impl CustomMeta {
         }
     }
 
+    pub fn is_registered(name: &str) -> bool {
+        assert_initialized_main_thread!();
+        unsafe { name.run_with_gstr(|name| !ffi::gst_meta_get_info(name.as_ptr()).is_null()) }
+    }
+
     #[doc(alias = "gst_buffer_add_custom_meta")]
     pub fn add<'a>(
         buffer: &'a mut BufferRef,