48 kbit/s MDCT Stereo: Concealment Artifact for Transition to TCX5

We just stumbled across another concealment artifact in the high-rate TCX core, when TCX5 happens in the next good frame after the concealed frame.

It seems to be related to the update of hTcxCfg->tcx_last_overlap_mode in getTCXWindowing(), which causes discontinuities in the transition:

    if ( element_mode != EVS_MONO && core == TCX_10_CORE )
    {
        /* also read last overlap */
        overlap_code = 0;
 
        if ( get_next_indice( st0, 1 ) )
        {
            overlap_code = 2 + get_next_indice( st0, 1 );
        }
 
        hTcxCfg->tcx_last_overlap_mode = overlap_code;
    }