vc2enc: allocate the DWT context with the current plane size

Previously used the luma size only.
Signed-off-by: Rostislav Pehlivanov <atomnuker@gmail.com>

libavcodec/vc2enc.c

@@ -1142,8 +1142,8 @@ static av_cold int vc2_encode_init(AVCodecContext *avctx)
 
         /* DWT init */
         if (ff_vc2enc_init_transforms(&s->transform_args[i].t,
-                                        s->plane[0].coef_stride,
-                                        s->plane[0].dwt_height))
+                                      s->plane[i].coef_stride,
+                                      s->plane[i].dwt_height))
             goto alloc_fail;
     }