PJ: remove back_scan.
PJ: remove copy_dwords/pj_copy_dwords.
PJ: remove exchange_words.
PJ: remove fcontrast.
PJ: remove pj_fsame.
PJ: remove xor_group.

Unused.  This functions more like a uint16 version of xor_bytes.
However, in V, xor_group worked on two arrays.  The comment in
rastlib/bymrast.c suggests it should be like the V function.
PJ: remove xor_words.