[NORMATIVE] S_FRAME handling update

This patch adds S-Frame implementation in the encoder, and fixes/improves
the functionality of S-Frames in the decoder.

Use the --sframe-dist=<arg> and --sframe-mode=<arg> switches to encode
using S-Frames.

*This patch allows combined inter prediction for S-Frames, in contrast
with the specification in the initial design document.*

BUG=aomedia:1459

Change-Id: Ia8b9dc9f7b97fcd435e79c62f2adc0e01f557627
diff --git a/av1/encoder/encoder.h b/av1/encoder/encoder.h
index c33c9ed..7a32c77 100644
--- a/av1/encoder/encoder.h
+++ b/av1/encoder/encoder.h
@@ -161,7 +161,9 @@
   // Key Framing Operations
   int auto_key;  // autodetect cut scenes and set the keyframes
   int key_freq;  // maximum distance to key frame.
-
+  int sframe_dist;
+  int sframe_mode;
+  int sframe_enabled;
   int lag_in_frames;  // how many frames lag before we start encoding
 
   // ----------------------------------------------------------------