[NORMATIVE] Enable mfmv for low latency settings

Tested in the low latency setting. Comparing the three options:

(1). Cut off temporal mv altogether.
(2). Use collocated mv referncing.
(3). Use mfmv referencing.

The current baseline is using (1).

The results are:
(2) improves the compression performance over (1) by -0.59% for
lowres and -0.65% for midres.

(3) improves over (1) by -1.05% for lowres and -1.43% for midres

BUG=aomedia:1403

Change-Id: Ie683110ecbe16131f8aea123bca490f8897ebdd8

av1/common/mvref_common.c

diff --git a/av1/common/mvref_common.c b/av1/common/mvref_common.c
index 4936c2c..67c26c0 100644
--- a/av1/common/mvref_common.c
+++ b/av1/common/mvref_common.c
@@ -1731,6 +1731,7 @@
   int gld_buf_idx = cm->frame_refs[GOLDEN_FRAME - LAST_FRAME].idx;
   int alt_buf_idx = cm->frame_refs[ALTREF_FRAME - LAST_FRAME].idx;
   int lst_buf_idx = cm->frame_refs[LAST_FRAME - LAST_FRAME].idx;
+  int lst2_buf_idx = cm->frame_refs[LAST2_FRAME - LAST_FRAME].idx;
   int bwd_buf_idx = cm->frame_refs[BWDREF_FRAME - LAST_FRAME].idx;
   int alt2_buf_idx = cm->frame_refs[ALTREF2_FRAME - LAST_FRAME].idx;
 
@@ -1782,6 +1783,12 @@
 
   if (alt_frame_index > cur_frame_index && ref_stamp >= 0)
     if (motion_field_projection(cm, ALTREF_FRAME, ref_stamp, 0)) --ref_stamp;
+
+  if (ref_stamp >= 0 && lst_buf_idx >= 0)
+    if (motion_field_projection(cm, LAST_FRAME, ref_stamp, 2)) --ref_stamp;
+
+  if (ref_stamp >= 0 && lst2_buf_idx >= 0)
+    if (motion_field_projection(cm, LAST2_FRAME, ref_stamp, 2)) --ref_stamp;
 }
 #endif  // CONFIG_MFMV