Bugfix for selective_ref_frame speed feature.

For speed >=3 this feature was set to level '3'. But at times, it was
triggered only at level == 2.
We now trigger it for level >= 2 instead.

Also, there is nothing special about level 3, so we remove that level.

Thanks to Debargha for finding this discrepancy.

Faster encoding at speed = 3 and above:
- speed 3: ~9% faster
- speed 4: ~9.5% faster

BDRate (PSNR) is roughly neutral (< +0.1%) on lowres, midres sets
for speed 3 and 4.

BUG=aomedia:1679

Change-Id: I58cdf1d00abb6558004f3015a20d71e8dfa983bd

av1/encoder/rdopt.c

diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index 0dbe335..3681095 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -9656,7 +9656,7 @@
   }
 
   if (sf->selective_ref_frame) {
-    if (sf->selective_ref_frame == 2 || x->cb_partition_scan) {
+    if (sf->selective_ref_frame >= 2 || x->cb_partition_scan) {
       if (ref_frame[0] == ALTREF2_FRAME || ref_frame[1] == ALTREF2_FRAME)
         if (get_relative_dist(
                 cm, cm->cur_frame->ref_frame_offset[ALTREF2_FRAME - LAST_FRAME],

av1/encoder/speed_features.c

diff --git a/av1/encoder/speed_features.c b/av1/encoder/speed_features.c
index c3e169c..2e25e81 100644
--- a/av1/encoder/speed_features.c
+++ b/av1/encoder/speed_features.c
@@ -193,7 +193,6 @@
   }
 
   if (speed >= 3) {
-    sf->selective_ref_frame = 3;
     sf->tx_size_search_method = boosted ? USE_FULL_RD : USE_LARGESTALL;
     sf->less_rectangular_check = 1;
     sf->mode_skip_start = 10;

av1/encoder/speed_features.h

diff --git a/av1/encoder/speed_features.h b/av1/encoder/speed_features.h
index df15e1b..dc89058 100644
--- a/av1/encoder/speed_features.h
+++ b/av1/encoder/speed_features.h
@@ -377,9 +377,9 @@
   // Used if partition_search_type = FIXED_SIZE_PARTITION
   BLOCK_SIZE always_this_block_size;
 
-  // Drop less likely picked reference frames in the RD search
-  // Has three levels for now: 1, 2 and 3, where higher levels are more
-  // aggressive than lower ones.
+  // Drop less likely to be picked reference frames in the RD search.
+  // Has three levels for now: 0, 1 and 2, where higher levels prune more
+  // aggressively than lower ones. (0 means no pruning).
   int selective_ref_frame;
 
   // Prune extended partition types search