Drop mvs with magnitude above 4096 from mvs reference frame

When either component of a motion vector is above 4096, drop this
motion vector from the motion vector reference frame for later
motion field projection use. The coding performance change is close
to 0 for lowres and midres. This ensures that the motion vector
and reference frame tuple can be efficiently stored within 32 bits.

Change-Id: I9ae60a5caab2d3f49200abb5415532d82986839f

av1/common/mvref_common.c

@@ -45,6 +45,9 @@ void av1_copy_frame_mvs(const AV1_COMMON *const cm, MODE_INFO *mi, int mi_row,
         if (ref_frame > INTRA_FRAME) {
           int8_t ref_idx = cm->ref_frame_side[ref_frame];
           if (ref_idx < 0) continue;
+          if ((abs(mi->mbmi.mv[idx].as_mv.row) > REFMVS_LIMIT) ||
+              (abs(mi->mbmi.mv[idx].as_mv.col) > REFMVS_LIMIT))
+            continue;
           mv->ref_frame[ref_idx] = ref_frame;
           mv->mv[ref_idx].as_int = mi->mbmi.mv[idx].as_int;
         }

av1/common/mvref_common.h

@@ -22,6 +22,12 @@ extern "C" {
 #define MVREF_ROWS 3
 #define MVREF_COLS 4
 
+// Set the upper limit of the motion vector component magnitude.
+// This would make a motion vector fit in 26 bits. Plus 3 bits for the
+// reference frame index. A tuple of motion vector can hence be stored within
+// 32 bit range for efficient load/store operations.
+#define REFMVS_LIMIT (1 << 12)
+
 typedef struct position {
   int row;
   int col;