Add experiment ext_skip

This coding tool is to introduce a new prediction mode for the
bi-predictive frames that have a forward referernce within 2 frames
away (distance denoted as 'fwd_delta'), and a backward reference,
within (3-fwd_delta) frames away.

If this prediction mode, namely 'ext_skip' is set, it will be coded
using compound prediction with the most recent forward and backward
reference frames as its reference pair, NEARESTMV as its motion mode,
and the skip flag is set for the residue.

Change-Id: I826034ccf1a956f4b350f0bc2e2dca8ea71b5197

build/cmake/aom_config_defaults.cmake

diff --git a/build/cmake/aom_config_defaults.cmake b/build/cmake/aom_config_defaults.cmake
index 157f9c9..93f7dca 100644
--- a/build/cmake/aom_config_defaults.cmake
+++ b/build/cmake/aom_config_defaults.cmake
@@ -142,6 +142,7 @@
 set(CONFIG_EXT_PARTITION_TYPES 0 CACHE NUMBER "AV1 experiment flag.")
 set(CONFIG_EXT_PARTITION_TYPES_AB 0 CACHE NUMBER "AV1 experiment flag.")
 set(CONFIG_EXT_REFS 1 CACHE NUMBER "AV1 experiment flag.")
+set(CONFIG_EXT_SKIP 0 CACHE NUMBER "AV1 experiment flag.")
 set(CONFIG_EXT_TILE 0 CACHE NUMBER "AV1 experiment flag.")
 set(CONFIG_EXT_TX 1 CACHE NUMBER "AV1 experiment flag.")
 set(CONFIG_FILTER_INTRA 0 CACHE NUMBER "AV1 experiment flag.")

configure

diff --git a/configure b/configure
index f12144e..4a3229a 100755
--- a/configure
+++ b/configure
@@ -345,6 +345,7 @@
     frame_marker
     jnt_comp
     frame_sign_bias
+    ext_skip
     obu
     amvr
     lpf_sb
@@ -581,6 +582,8 @@
     enabled jnt_comp && enable_feature frame_marker
     enabled frame_sign_bias && enable_feature frame_marker
     enabled txmg && enable_feature highbitdepth
+    enabled ext_skip && enable_feature frame_marker
+    enabled ext_skip && enable_feature ext_refs
 
     if enabled rawbits && enabled ans; then
       log_echo "rawbits requires not ans, so disabling rawbits"