resample test

    // convert from planar int16 to interleaved float32
    // also change sample rate from 8kHz to 16kHz.
    const int16_t in[2][4] = {{0,1,2,3}, {10, 9, 8, 7}};
    const uint8_t ** in_ptr = (const uint8_t**)in;
    float out[16] = {0};
    uint8_t * out_ptr = (uint8_t*)out;

    swr_context = swr_alloc_set_opts(NULL,
                        AV_CH_LAYOUT_STEREO, AV_SAMPLE_FMT_S16P, 8000,
                        AV_CH_LAYOUT_STEREO, AV_SAMPLE_FMT_FLT, 16000,
                        0, NULL);
    int res = swr_init(swr_context);
    res = swr_convert(swr_context, &out_ptr, 8, in_ptr, 4);

    // i have a debug point here. context seems fine,
    // but out always contains just 0,0,0,...
    // in remains unmodified.
    printf("DONE");