Integrate a new LZ77 looking for matches in the neighborhood of a pixel only.

Change-Id: Ie2bbfee0a8d154b58f4a3068f3f634b7dad4c12d

src/enc/backward_references_cost_enc.c

@@ -648,7 +648,7 @@ static int BackwardReferencesHashChainDistanceOnly(
           // better than whatever we add.
           int offset_j, len_j = 0;
           int j;
-          assert(len == MAX_LENGTH);
+          assert(len == MAX_LENGTH || len == pix_count - i);
           // Figure out the last consecutive pixel within [i, reach + 1] with
           // the same offset.
           for (j = i; j <= reach; ++j) {