Support backward motion vector projection from alt2

Support backward projection of the motion vectors from the
ALTREF2 reference frame to build the motion field.

Change-Id: I81d41c3ea71c14e6d8932f4e106c34976696b74d

av1/common/mvref_common.c

@@ -1970,7 +1970,7 @@ void av1_setup_motion_field(AV1_COMMON *cm) {
   }
 
   motion_field_projection(cm, ALTREF_FRAME);
-
+  motion_field_projection(cm, ALTREF2_FRAME);
   motion_field_projection(cm, BWDREF_FRAME);
 }
 #endif  // CONFIG_MFMV