Transmit DisplayFrameSync message, handle in WaterfallSpectrum.

More kludginess, especially around initialization and timing. But it addresses the flickering lines of pixels at the bottom of the waterfall scroll area!

firmware/common/message.hpp

@@ -45,7 +45,7 @@ public:
 		RSSIStatistics = 0,
 		BasebandStatistics = 1,
 		ChannelStatistics = 2,
-
+		DisplayFrameSync = 3,
 		AudioStatistics = 4,
 		BasebandConfiguration = 5,
 		TPMSPacket = 6,
@@ -133,6 +133,14 @@ public:
 	ChannelStatistics statistics;
 };
 
+class DisplayFrameSyncMessage : public Message {
+public:
+	constexpr DisplayFrameSyncMessage(
+	) : Message { ID::DisplayFrameSync }
+	{
+	}
+};
+
 struct AudioStatistics {
 	int32_t rms_db;
 	int32_t max_db;