Flexible TD renderer filter memory allocation

In ivas_stat_rend.h there is the ToDo:

float hrf_left_prev[SFX_SPAT_BIN_MAX_FILTER_LENGTH]; /* Todo: Should we allocate these buffers with malloc() instead of the maximum length? */

In short, the buffers should be allocated using malloc/free to make memory handling more efficient and also allow longer filters when loaded from file.