Fix the sub8x8 block inter prediction with scaled reference frame

Sync the encoder's buffer offset calculation for sub8x8 block motion
compensated prediction with scaled reference frame to match the
decoder's behavior. This resolves an enc/dec mismatch issue when
sub8x8 inter mode with scaled is turned on.

Change-Id: I4bab3672b007a5ae0c992f8a701341892d2458b0

vp9/common/vp9_reconinter.c

@@ -187,7 +187,19 @@ static void build_inter_predictors(MACROBLOCKD *xd, int plane, int block,
     const int is_scaled = vp9_is_scaled(sf);
 
     if (is_scaled) {
-      pre = pre_buf->buf + scaled_buffer_offset(x, y, pre_buf->stride, sf);
+      // Co-ordinate of containing block to pixel precision.
+      const int x_start = (-xd->mb_to_left_edge >> (3 + pd->subsampling_x));
+      const int y_start = (-xd->mb_to_top_edge >> (3 + pd->subsampling_y));
+      if (plane == 0)
+        pre_buf->buf = xd->block_refs[ref]->buf->y_buffer;
+      else if (plane == 1)
+        pre_buf->buf = xd->block_refs[ref]->buf->u_buffer;
+      else
+        pre_buf->buf = xd->block_refs[ref]->buf->v_buffer;
+
+      pre_buf->buf += scaled_buffer_offset(x_start + x, y_start + y,
+                                           pre_buf->stride, sf);
+      pre = pre_buf->buf;
       scaled_mv = vp9_scale_mv(&mv_q4, mi_x + x, mi_y + y, sf);
       xs = sf->x_step_q4;
       ys = sf->y_step_q4;