ivas_dec() / ivas_jbm_dec() harmonization

The names and handling of output buffers in ivas_dec() resp. ivas_jbm_dec() is chaotic and would benefit from harmonization.

1. In ivas_dec(), there is:

    for ( n = 0; n < MAX_OUTPUT_CHANNELS + MAX_NUM_OBJECTS; n++ )
    {
        output[n] = st_ivas->p_output_f[n];
        p_output[n] = output[n];
    }

I propose to get rid of output[n] and use p_output[n] instead in the whole function.

2. In ivas_jbm_dec(), there is:

    for ( n = 0; n < MAX_TRANSPORT_CHANNELS; n++ )
    {
        output[n] = p_output_f[n];
        p_output[n] = &output[n][0];
    }

Similarly, I propose to use p_output[n] in the whole function.

At the same time, I propose to introduce a new function ivas_apply_non_diegetic_panning() which replaces four identical code blocks.

Finally, several return error codes were added to several instances.