fast: Replace the fetcher_info_t table with a pixman_iter_info_t table

fast: Replace the fetcher_info_t table with a pixman_iter_info_t table

Similar to the SSE2 and MMX patches, this commit replaces a table of
fetcher_info_t with a table of pixman_iter_info_t, and similar to the
noop patch, both fast_src_iter_init() and fast_dest_iter_init() are
now doing exactly the same thing, so their code can be shared in a new
function called fast_iter_init_common().