error_concealment: Use previous pictures motion vectors when the current ones have been lost.

Looks better for some cases, worse for others, overall not much difference.
Its more correct though.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/error_resilience.c

@@ -381,6 +381,14 @@ static void guess_mv(MpegEncContext *s){
         fixed[mb_xy]= f;
         if(f==MV_FROZEN)
             num_avail++;
+        else if(s->last_picture.data[0]){
+            const int mb_y= mb_xy / s->mb_stride;
+            const int mb_x= mb_xy % s->mb_stride;
+            const int mot_index= (mb_x + mb_y*mot_stride) * mot_step;
+            s->current_picture.motion_val[0][mot_index][0]= s->last_picture.motion_val[0][mot_index][0];
+            s->current_picture.motion_val[0][mot_index][1]= s->last_picture.motion_val[0][mot_index][1];
+            s->current_picture.ref_index[0][4*mb_xy]      = s->last_picture.ref_index[0][4*mb_xy];
+        }
     }
 
     if((!(s->avctx->error_concealment&FF_EC_GUESS_MVS)) || num_avail <= mb_width/2){

tests/ref/vsynth1/error

 7416dfd319f04044d4575dc9d1b406e1 *./tests/data/vsynth1/error-mpeg4-adv.avi
 756836 ./tests/data/vsynth1/error-mpeg4-adv.avi
-ef8bfcd6e0883daba95d0f32486ebe2d *./tests/data/error.vsynth1.out.yuv
-stddev:   18.05 PSNR: 23.00 MAXDIFF:  245 bytes:  7603200/  7603200
+79e94ba32b37759397362cbcb479d4d3 *./tests/data/error.vsynth1.out.yuv
+stddev:   18.36 PSNR: 22.85 MAXDIFF:  243 bytes:  7603200/  7603200

tests/ref/vsynth2/error

 90e65096aa9ebafa3fe3f44a5a47cdc4 *./tests/data/vsynth2/error-mpeg4-adv.avi
 176588 ./tests/data/vsynth2/error-mpeg4-adv.avi
-9fe1082179f80179439953c7397a46ef *./tests/data/error.vsynth2.out.yuv
-stddev:    9.00 PSNR: 29.04 MAXDIFF:  168 bytes:  7603200/  7603200
+96baa9e4c24c837a3ba5abd8dd2cdd30 *./tests/data/error.vsynth2.out.yuv
+stddev:    8.98 PSNR: 29.06 MAXDIFF:  184 bytes:  7603200/  7603200