Add min_tx_size variable to recursive transform block partition system

Replace max_tx_size with min_tx_size for transform type decision.

Change-Id: I64e39923a67903d52b381bd93eaac33b3400a201
diff --git a/av1/common/blockd.h b/av1/common/blockd.h
index 1d08cfa..8649b7d 100644
--- a/av1/common/blockd.h
+++ b/av1/common/blockd.h
@@ -189,6 +189,7 @@
   // TODO(jingning): This effectively assigned a separate entry for each
   // 8x8 block. Apparently it takes much more space than needed.
   TX_SIZE inter_tx_size[MAX_MIB_SIZE][MAX_MIB_SIZE];
+  TX_SIZE min_tx_size;
 #endif
   int8_t skip;
   int8_t segment_id;