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;

jcdctmgr.c

@@ -1520,7 +1520,7 @@ quantize_trellis_arith(j_compress_ptr cinfo, arith_rates *r, JBLOCKROW coef_bloc
             if (v2 >>= 1) {
               coef_bits += r->rate_ac[st][1];
               m <<= 1;
-              st = 189; /* TODO: condition 189/217 */
+              st = (i <= r->arith_ac_K) ? 189 : 217;
               while (v2 >>= 1) {
                 coef_bits += r->rate_ac[st][1];
                 m <<= 1;

jpegint.h

@@ -110,6 +110,7 @@ struct jpeg_comp_master {
 typedef struct {
   float rate_dc[DC_STAT_BINS][2];
   float rate_ac[AC_STAT_BINS][2];
+  int arith_ac_K;
 } arith_rates;
 
 /* Main buffer control (downsampled-data buffer) */