Restore the current preview when reopening stream controls

tools/chatgui/src/stream_publish_engine.cpp

@@ -133,6 +133,13 @@ bool StreamPublishEngine::active() const { return m->id != 0; }
 bool StreamPublishEngine::running() const { return m->running.load(); }
 uint64_t StreamPublishEngine::group() const { return m->group; }
 StreamPublishConfig StreamPublishEngine::config() const { std::lock_guard<std::mutex> guard(m->lock); return m->wanted; }
+QVariant StreamPublishEngine::previewFrame() const {
+#ifdef HAVE_STREAM_CAPTURE
+    std::lock_guard<std::mutex> guard(m->lock); return QVariant::fromValue(m->previewFrame);
+#else
+    return {};
+#endif
+}
 QList<int> StreamPublishEngine::codecs() const { std::lock_guard<std::mutex> guard(m->lock); return m->codecs; }
 bool StreamPublishEngine::muted() const { return m->muted.load(); }
 void StreamPublishEngine::setMuted(bool muted) {