scrollrect
int (*scrollrect)(VTermRect rect, int downward, int rightward, void *user);
scrollrect(rect, first_blank_row - new_rows, 0, user);
scrollrect(rect, -1, 0, user);
.scrollrect = &scrollrect,
if(state->callbacks && state->callbacks->scrollrect)
if((*state->callbacks->scrollrect)(rect, downward, rightward, state->cbdata))