Fill refpoc even if use_ref_frame_mvs=0.

Fixes #60.

src/decode.c

@@ -2929,10 +2929,11 @@ int dav1d_submit_frame(Dav1dContext *const c) {
         f->mvs_ref = dav1d_ref_create(f->sb128h * 32 * f->b4_stride *
                                       sizeof(*f->mvs));
         f->mvs = f->mvs_ref->data;
+        for (int i = 0; i < 7; i++)
+            f->refpoc[i] = f->refp[i].p.poc;
         if (f->frame_hdr.use_ref_frame_mvs) {
             for (int i = 0; i < 7; i++) {
                 const int refidx = f->frame_hdr.refidx[i];
-                f->refpoc[i] = f->refp[i].p.poc;
                 if (c->refs[refidx].refmvs != NULL &&
                     f->refp[i].p.p.w == f->cur.p.p.w &&
                     f->refp[i].p.p.h == f->cur.p.p.h)