With some attempt at vsync prediction, seeks to smooth audio/video output.

There's plenty more work to do here, but hopefully it takes the issue immediately off the table.

ClockReceiver/VSyncPredictor.hpp

@@ -0,0 +1,63 @@
+//
+//  VSyncPredictor.hpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 14/06/2020.
+//  Copyright © 2020 Thomas Harte. All rights reserved.
+//
+
+#ifndef VSyncPredictor_hpp
+#define VSyncPredictor_hpp
+
+#include "TimeTypes.hpp"
+
+namespace Time {
+
+/*!
+	For platforms that provide no avenue into vsync tracking other than block-until-sync,
+	this class tracks: (i) how long frame draw takes; and (ii) the apparent frame period; in order
+	to suggest when you should next start drawing.
+*/
+class VSyncPredictor {
+	public:
+		void begin_redraw() {
+			redraw_begin_time_ = nanos_now();
+		}
+
+		void end_redraw() {
+			const auto redraw_length = nanos_now() - redraw_begin_time_;
+			redraw_period_ =
+				((redraw_period_ * 9) +
+				((redraw_length) * 1)) / 10;
+		}
+
+		void announce_vsync() {
+			const auto vsync_time = nanos_now();
+			if(last_vsync_) {
+				// Use an IIR to try to converge on frame times.
+				vsync_period_ =
+					((vsync_period_ * 9) +
+					((vsync_time - last_vsync_) * 1)) / 10;
+			}
+			last_vsync_ = vsync_time;
+		}
+
+		Nanos suggested_draw_time() {
+			// TODO: this is a very simple version of how this calculation
+			// should be made. It's tracking the average amount of time these
+			// things take, therefore will often be wrong. Deviations need to
+			// be accounted for.
+			return last_vsync_ + vsync_period_ - redraw_period_;
+		}
+
+	private:
+		Nanos redraw_begin_time_ = 0;
+		Nanos last_vsync_ = 0;
+
+		Nanos vsync_period_ = 1'000'000'000 / 60;	// Seems like a good first guess.
+		Nanos redraw_period_ = 0;
+};
+
+}
+
+#endif /* VSyncPredictor_hpp */