main external API structure.
void(* diff_int16)(uint16_t *dst, const uint16_t *src1, const uint16_t *src2, unsigned mask, int w)
void(* sub_hfyu_median_pred_int16)(uint16_t *dst, const uint16_t *src1, const uint16_t *src2, unsigned mask, int w, int *left, int *left_top)