ext-inter: Use joint_motion_search for masked compounds

Add functions which take both components of a masked compound and
compute the resulting SAD/SSE. Extend joint_motion_search to understand
masked compounds, and use it to evaluate NEW_NEWMV modes.

Change-Id: I782199a20d119a6c61c6567df157508125ac7ce7
diff --git a/av1/encoder/mbgraph.c b/av1/encoder/mbgraph.c
index 8578611..3f5daeb 100644
--- a/av1/encoder/mbgraph.c
+++ b/av1/encoder/mbgraph.c
@@ -52,11 +52,14 @@
   {
     int distortion;
     unsigned int sse;
-    cpi->find_fractional_mv_step(x, ref_mv, cpi->common.allow_high_precision_mv,
-                                 x->errorperbit, &v_fn_ptr, 0,
-                                 mv_sf->subpel_iters_per_step,
-                                 cond_cost_list(cpi, cost_list), NULL, NULL,
-                                 &distortion, &sse, NULL, 0, 0, 0);
+    cpi->find_fractional_mv_step(
+        x, ref_mv, cpi->common.allow_high_precision_mv, x->errorperbit,
+        &v_fn_ptr, 0, mv_sf->subpel_iters_per_step,
+        cond_cost_list(cpi, cost_list), NULL, NULL, &distortion, &sse, NULL,
+#if CONFIG_EXT_INTER
+        NULL, 0, 0,
+#endif
+        0, 0, 0);
   }
 
 #if CONFIG_EXT_INTER