from jcmainct.c:28
typedef struct
{
struct jpeg_c_main_controller pub;
JDIMENSION cur_iMCU_row;
JDIMENSION rowgroup_ctr;
boolean suspended;
J_BUF_MODE pass_mode;
JSAMPARRAY buffer[MAX_COMPONENTS];
#ifdef FULL_MAIN_BUFFER_SUPPORTED
jvirt_sarray_ptr whole_image[MAX_COMPONENTS];
#endif
} my_main_controller;