avcodec/mpegvideo: Fix block height for lowres 3 interlaced blocks

Fixes green trash
Fixes part of Ticket2535

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mpegvideo.c

@@ -2349,11 +2349,12 @@ static av_always_inline void mpeg_motion_lowres(MpegEncContext *s,
     pix_op[lowres - 1](dest_y, ptr_y, linesize, h, sx, sy);
 
     if (!CONFIG_GRAY || !(s->flags & CODEC_FLAG_GRAY)) {
+        int hc = s->chroma_y_shift ? (h+1-bottom_field)>>1 : h;
         uvsx = (uvsx << 2) >> lowres;
         uvsy = (uvsy << 2) >> lowres;
-        if (h >> s->chroma_y_shift) {
-            pix_op[op_index](dest_cb, ptr_cb, uvlinesize, h >> s->chroma_y_shift, uvsx, uvsy);
-            pix_op[op_index](dest_cr, ptr_cr, uvlinesize, h >> s->chroma_y_shift, uvsx, uvsy);
+        if (hc) {
+            pix_op[op_index](dest_cb, ptr_cb, uvlinesize, hc, uvsx, uvsy);
+            pix_op[op_index](dest_cr, ptr_cr, uvlinesize, hc, uvsx, uvsy);
         }
     }
     // FIXME h261 lowres loop filter