Refine rate estimate in trellis

Take into account cutoff parameter to switch between sets of contexts
for rate estimation in arithmetic coding version of trellis quantization

jcarith.c

@@ -940,6 +940,9 @@ jget_arith_rates (j_compress_ptr cinfo, int dc_tbl_no, int ac_tbl_no, arith_rate
 {
   int i;
   arith_entropy_ptr entropy = (arith_entropy_ptr) cinfo->entropy;
+  
+  r->arith_ac_K = cinfo->arith_ac_K[ac_tbl_no];
+  
   for (i = 0; i < DC_STAT_BINS; i++) {
     int state = entropy->dc_stats[dc_tbl_no][i];
     int mps_val = state >> 7;