audio: master-clock playback timing, output clock compensation, buffer config, interpolated speed

- playback timer uses the audio output device as its master clock: the
  PortAudio callback counts consumed frames (including underrun
  zero-fill) so video cannot drift away from what is heard; wall clock
  remains as fallback when no clocked output is running
- output clock compensates for device output latency; new Preferences >
  Audio buffer size setting (0 = auto)
- SampleBuffer::speed() now uses linear interpolation instead of
  nearest-neighbor sampling
- regression tests: audio-clock driven timer (fwd/rev/speed), wall
  clock fallback, interpolation correctness

app/widget/viewer/viewerplaybacktimer.cpp

@@ -28,16 +28,36 @@ namespace olive
 
 void ViewerPlaybackTimer::start(const int64_t &start_timestamp,
 								const int &playback_speed,
-								const double &timebase)
+								const double &timebase,
+								const PlaybackAudioClock *audio_clock)
 {
 	timer_.start();
 	start_timestamp_ = start_timestamp;
 	playback_speed_ = playback_speed;
 	timebase_ = timebase * 1000;
+	audio_clock_ = audio_clock;
 }
 
 int64_t ViewerPlaybackTimer::get_timestamp_now() const
 {
+	// The audio output clock is the master clock when available: sound card
+	// consumption is what the viewer must stay in sync with, and unlike the
+	// wall clock it cannot drift away from what is actually heard
+	if (audio_clock_) {
+		const double audio_seconds = audio_clock_->seconds();
+		if (audio_seconds >= 0.0) {
+			// At speeds other than 1x the audio tempo is scaled, so one
+			// output second corresponds to |speed| timeline seconds. The
+			// result is already in timeline frames, so it is applied
+			// signed rather than multiplied by the speed again.
+			const double timeline_ms =
+				audio_seconds * 1000.0 * qAbs(playback_speed_);
+			const int64_t frames_since_start = qFloor(timeline_ms / timebase_);
+			return start_timestamp_ +
+				frames_since_start * (playback_speed_ < 0 ? -1 : 1);
+		}
+	}
+
 	int64_t real_time = timer_.elapsed();
 
 	int64_t frames_since_start =