JNT_COMP: 1. Init version of experiment JNT_COMP

Enable to assign distance based weight for joint compound prediction.

(w0, w1) are weights for two predictors of different distance to
current frame.

Use 4 bit precision for quantized distance weight. e.g.
the prediction is generated as

value = (w0 * p0 + w1 * p1) >> n
w0 + w1 = (1 << n), n = 4;

Change-Id: Ib0ff0c41c82b9ebb033f498e90c18a03d18969e4
diff --git a/av1/common/convolve.h b/av1/common/convolve.h
index c43f649..61f290a 100644
--- a/av1/common/convolve.h
+++ b/av1/common/convolve.h
@@ -35,6 +35,10 @@
   int round_1;
   int plane;
   int do_post_rounding;
+#if CONFIG_JNT_COMP
+  int fwd_offset;
+  int bck_offset;
+#endif
 } ConvolveParams;
 
 static INLINE ConvolveParams get_conv_params(int ref, int do_average,