归一混音pcm，直接使用

void Mix_Audio(char** src_data,
                                  char* mix_data,
                                  int channels,
                                  int buffer_size) {
  //归一化混音
  int const MAX = 32767;
  int const MIN = -32768;

  double f = 1;
  int output;
  int i = 0, j = 0;

  for (i = 0; i < buffer_size / channels; i++) {
    int temp = 0;
    for (j = 0; j < channels; j++) {
      //两个文件对应音轨值相加
      temp += *(short*)(src_data[j] + i * channels);
    }

    output = (int)(temp * f);

    if (output > MAX) {
      f = (double)MAX / (double)(output);
      output = MAX;
    }
    if (output < MIN) {
      f = (double)MIN / (double)(output);
      output = MIN;
    }
    if (f < 1) {