gstreamer-app: Assert that GStreamer is initialized before creating instances

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1184>

gstreamer-app/src/app_sink.rs

@@ -271,6 +271,7 @@ impl AppSink {
     ///
     /// This method returns an instance of [`AppSinkBuilder`](crate::builders::AppSinkBuilder) which can be used to create [`AppSink`] objects.
     pub fn builder() -> AppSinkBuilder {
+        assert_initialized_main_thread!();
         AppSinkBuilder::default()
     }
 

gstreamer-app/src/app_src.rs

@@ -216,6 +216,7 @@ impl AppSrc {
     ///
     /// This method returns an instance of [`AppSrcBuilder`](crate::builders::AppSrcBuilder) which can be used to create [`AppSrc`] objects.
     pub fn builder() -> AppSrcBuilder {
+        assert_initialized_main_thread!();
         AppSrcBuilder::default()
     }