Only recompute level_cost_[] when needed

Add a dirty_ flag to keep track of updated probabilities and the need to
recompute the level costs.
This only makes a difference for "-m 2" method which was sub-optimal.
But it's overall cleaner to have this flag.

Change-Id: I21c71201e1d07a923d97a3adf2fbbd7d67d35433

src/enc/vp8enci.h

@@ -188,6 +188,7 @@ typedef struct {
   ProbaArray coeffs_[NUM_TYPES][NUM_BANDS];      // 924 bytes
   StatsArray stats_[NUM_TYPES][NUM_BANDS];       // 4224 bytes
   CostArray level_cost_[NUM_TYPES][NUM_BANDS];   // 11.4k
+  int dirty_;               // if true, need to call VP8CalculateLevelCosts()
   int use_skip_proba_;      // Note: we always use skip_proba for now.
   int nb_skip_;             // number of skipped blocks
 } VP8Proba;