decklink-out: preroll frames before StartScheduledPlayback, detect output format

nodes/decklink-out/src/decklink_out_node.hpp

@@ -55,6 +55,8 @@ private:
     IDeckLinkOutput* output_ = nullptr;
     BMDTimeValue frame_duration_ = 1000;
     BMDTimeScale time_scale_ = 50000;
+    BMDPixelFormat output_format_ = bmdFormat10BitYUV;
+    int preroll_frames_ = 3;
 
     std::atomic<bool> playing_{false};
     int create_fail_count_ = 0;