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/config/config.cpp

@@ -210,6 +210,10 @@ void Config::set_defaults()
 		QStringLiteral("AudioOutputSampleFormat"), NodeValue::k_text,
 		QString::fromStdString(SampleFormat(SampleFormat::s16).to_string()));
 
+	// Output buffer size in frames, 0 = let PortAudio/the device decide
+	set_entry_internal(QStringLiteral("AudioOutputBufferSize"), NodeValue::k_int,
+					 0);
+
 	set_entry_internal(QStringLiteral("AudioRecordingFormat"), NodeValue::k_int,
 					 ExportFormat::k_format_wav);
 	set_entry_internal(QStringLiteral("AudioRecordingCodec"), NodeValue::k_int,