gst-utils: prevent dead lock when requesting key unit

Sending the UpstreamForceKeyUnitEvent using gst_element_send_event()
internally takes the state lock. If appsink is pre-rolling we are also
holding the preroll lock.

This may result in a dead lock with the thread doing the state change as
this one takes the state lock and then the pre-roll lock.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1316>

gstreamer-utils/src/streamproducer.rs

@@ -320,7 +320,9 @@ impl StreamProducer {
         drop(consumers);
 
         if needs_keyframe_request {
-            appsink.send_event(
+            // Do not use `gst_element_send_event()` as it takes the state lock which may lead to dead locks.
+            let pad = appsink.static_pad("sink").unwrap();
+            pad.push_event(
                 gst_video::UpstreamForceKeyUnitEvent::builder()
                     .all_headers(true)
                     .build(),