Add one more level for selective ref frame speed feature

Change-Id: Idb4c986d8cd795e2d2ea76023674c8b2ba5d353c

av1/encoder/speed_features.c

@@ -203,6 +203,8 @@ static void set_good_speed_features_framesize_independent(AV1_COMP *cpi,
   }
 
   if (speed >= 3) {
+    sf->selective_ref_frame = 3;
+
     sf->tx_size_search_method =
         frame_is_boosted(cpi) ? USE_FULL_RD : USE_LARGESTALL;
     sf->mode_search_skip_flags =

av1/encoder/speed_features.h

@@ -335,7 +335,8 @@ typedef struct SPEED_FEATURES {
   BLOCK_SIZE always_this_block_size;
 
   // Drop less likely picked reference frames in the RD search
-  // Has two levels for now: 1 and 2, where 2 is more aggressive than 1.
+  // Has three levels for now: 1, 2 and 3, where higher levels are more
+  // aggressive than lower ones.
   int selective_ref_frame;
 
 #if CONFIG_EXT_PARTITION_TYPES