vp9: fix mvref finding to adhere to bug in libvpx.

Fixes a particular youtube video that I unfortunately can't share.

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/vp9mvs.c

@@ -191,11 +191,15 @@ static void find_ref_mvs(VP9Context *s,
             if (mv->ref[0] != ref && mv->ref[0] >= 0)
                 RETURN_SCALE_MV(mv->mv[0],
                                 s->signbias[mv->ref[0]] != s->signbias[ref]);
-            if (mv->ref[1] != ref && mv->ref[1] >= 0)
+            if (mv->ref[1] != ref && mv->ref[1] >= 0 &&
+                // BUG - libvpx has this condition regardless of whether
+                // we used the first ref MV and pre-scaling
+                AV_RN32A(&mv->mv[0]) != AV_RN32A(&mv->mv[1])) {
                 RETURN_SCALE_MV(mv->mv[1],
                                 s->signbias[mv->ref[1]] != s->signbias[ref]);
             }
         }
+    }
 
     // MV at this position in previous frame, using different reference frame
     if (s->use_last_frame_mvs) {
@@ -204,10 +208,14 @@ static void find_ref_mvs(VP9Context *s,
         if (mv->ref[0] != ref && mv->ref[0] >= 0)
             RETURN_SCALE_MV(mv->mv[0],
                             s->signbias[mv->ref[0]] != s->signbias[ref]);
-        if (mv->ref[1] != ref && mv->ref[1] >= 0)
+        if (mv->ref[1] != ref && mv->ref[1] >= 0 &&
+            // BUG - libvpx has this condition regardless of whether
+            // we used the first ref MV and pre-scaling
+            AV_RN32A(&mv->mv[0]) != AV_RN32A(&mv->mv[1])) {
             RETURN_SCALE_MV(mv->mv[1],
                             s->signbias[mv->ref[1]] != s->signbias[ref]);
         }
+    }
 
     AV_ZERO32(pmv);
 #undef INVALID_MV