fix decklink-out: init grains_read from current time to avoid epoch realignment

nodes/decklink-out/src/decklink_out_node.hpp

@@ -45,6 +45,7 @@ private:
     mxlRational output_rate_{50, 1};
     uint64_t read_index_ = 0;
     uint64_t grains_read_ = 0;
+    bool first_frame_ = true;
 
     int device_index_ = 0;
     BMDDisplayMode display_mode_ = bmdModeHD1080i50;