GstEnginePipeline: Fix possible race condition in pipeline destructor

Wait for ongoing state changes to complete before setting pipeline to NULL.
This prevents race conditions with async state transitions that can cause crashes in GStreamer elements.

Fixes #1875

src/engine/gstenginepipeline.h

@@ -385,6 +385,10 @@ class GstEnginePipeline : public QObject {
 
   mutex_protected<GstState> last_set_state_in_progress_;
   mutex_protected<GstState> last_set_state_async_in_progress_;
+
+  // Track futures for this pipeline's state changes to allow waiting for them in destructor
+  QList<QFuture<GstStateChangeReturn>> pending_state_changes_;
+  QMutex mutex_pending_state_changes_;
 };
 
 using GstEnginePipelinePtr = QSharedPointer<GstEnginePipeline>;