vc1dec: Set chroma reference field from REFFIELD for 1REF field pictures

Interlaced field pictures can have one or two reference pictures, signaled
by NUMREF syntax element. For single reference pictures, reference picture
is determined by REFFIELD syntax element.
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

libavcodec/vc1dec.c

@@ -794,6 +794,7 @@ static void vc1_mc_4mv_chroma(VC1Context *v, int dir)
     /* calculate chroma MV vector from four luma MVs */
     if (!v->field_mode || (v->field_mode && !v->numref)) {
         valid_count = get_chroma_mv(mvx, mvy, intra, 0, &tx, &ty);
+        chroma_ref_type = v->reffield;
         if (!valid_count) {
             s->current_picture.f.motion_val[1][s->block_index[0] + v->blocks_off][0] = 0;
             s->current_picture.f.motion_val[1][s->block_index[0] + v->blocks_off][1] = 0;