Update Windows ffmpeg to 3.2.4

Externals/ffmpeg/include/libavutil/audio_fifo.h

@@ -110,6 +110,23 @@ int av_audio_fifo_write(AVAudioFifo *af, void **data, int nb_samples);
  */
 int av_audio_fifo_peek(AVAudioFifo *af, void **data, int nb_samples);
 
+/**
+ * Peek data from an AVAudioFifo.
+ *
+ * @see enum AVSampleFormat
+ * The documentation for AVSampleFormat describes the data layout.
+ *
+ * @param af          AVAudioFifo to read from
+ * @param data        audio data plane pointers
+ * @param nb_samples  number of samples to peek
+ * @param offset      offset from current read position
+ * @return            number of samples actually peek, or negative AVERROR code
+ *                    on failure. The number of samples actually peek will not
+ *                    be greater than nb_samples, and will only be less than
+ *                    nb_samples if av_audio_fifo_size is less than nb_samples.
+ */
+int av_audio_fifo_peek_at(AVAudioFifo *af, void **data, int nb_samples, int offset);
+
 /**
  * Read data from an AVAudioFifo.
  *