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/viewer.cpp

@@ -1384,6 +1384,10 @@ void ViewerWidget::finish_play_preprocess()
 
 	int64_t playback_start_time = get_timestamp();
 
+	// Restart the audio output clock for this playback run; the playback
+	// timer uses it as its master clock
+	AudioManager::instance()->reset_output_clock();
+
 	// Start audio waveform playback
 	if (!prequeued_audio_.isEmpty()) {
 		QString error;