Add CONFIG_SIMPLE_BWD_ADAPT experiment.

Use just one tile for CDF backwards update.

Enable by default.

Change-Id: I0c89f87c16e5b24894f8cd3151efc5ad5aaecb49
diff --git a/configure b/configure
index dccc16b..45bffc5 100755
--- a/configure
+++ b/configure
@@ -355,6 +355,7 @@
     ext_warped_motion
     restrict_compressed_hdr
     horzonly_frame_superres
+    simple_bwd_adapt
 "
 CONFIG_LIST="
     dependency_tracking
@@ -545,6 +546,7 @@
       soft_enable ext_comp_refs
       soft_enable ext_delta_q
       soft_enable parallel_deblocking
+      soft_enable simple_bwd_adapt
     fi
 
     # Enable low-bitdepth pixel pipeline by default