Compound inter encoder bug fix.

In the longer term the encoder should allow compound as long
as one of the buffers has opposite sign bias and as per the decoder
this buffer is then set as the fixed reference. However at the moment
the encoder and RD loop only supports the case where the ALTREF_FRAME
buffer (or third of the 3 allowed in any given frame) is the odd one out.

This patch fixes a bug that would allow compound inter and set
fixed ref to ALTREF_FRAME when it is not the odd one out.

Change-Id: Ic83a69486e088a147ba83a4aedc2a0042f6b3721

vp9/encoder/vp9_encodeframe.c

@@ -1752,10 +1752,16 @@ static void reset_skip_txfm_size(VP9_COMP *cpi, TX_SIZE txfm_max) {
 void vp9_encode_frame(VP9_COMP *cpi) {
   VP9_COMMON *const cm = &cpi->common;
 
-  if (cm->ref_frame_sign_bias[LAST_FRAME] ==
+  // In the longer term the encoder should be generalized to match the
-      cm->ref_frame_sign_bias[GOLDEN_FRAME] &&
+  // decoder such that we allow compound where one of the 3 buffers has a
-      cm->ref_frame_sign_bias[LAST_FRAME] ==
+  // differnt sign bias and that buffer is then the fixed ref. However, this
-      cm->ref_frame_sign_bias[ALTREF_FRAME]) {
+  // requires further work in the rd loop. For now the only supported encoder
+  // side behaviour is where the ALT ref buffer has oppositie sign bias to
+  // the other two.
+  if ((cm->ref_frame_sign_bias[ALTREF_FRAME] ==
+       cm->ref_frame_sign_bias[GOLDEN_FRAME]) ||
+      (cm->ref_frame_sign_bias[ALTREF_FRAME] ==
+       cm->ref_frame_sign_bias[LAST_FRAME])) {
     cm->allow_comp_inter_inter = 0;
   } else {
     cm->allow_comp_inter_inter = 1;