int lowres
low resolution decoding, 1-> 1/2 size, 2->1/4 size 
int idct_algo
IDCT algorithm, see FF_IDCT_* below. 
void(* idct_add)(uint8_t *dest, int line_size, int16_t *block)
block -> idct -> add dest -> clip to unsigned 8 bit -> dest. 
void(* idct_put)(uint8_t *dest, int line_size, int16_t *block)
block -> idct -> clip to unsigned 8 bit -> dest. 
main external API structure. 
enum idct_permutation_type perm_type
void(* idct)(int16_t *block)