j2kenc: enable 4:2:0 YUV it works as "well" as the other subsampled yuv formats

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

libavcodec/j2kenc.c

@@ -1041,6 +1041,7 @@ AVCodec ff_jpeg2000_encoder = {
     .capabilities= CODEC_CAP_EXPERIMENTAL,
     .pix_fmts =
         (enum PixelFormat[]) {PIX_FMT_GRAY8, PIX_FMT_RGB24,
+                              PIX_FMT_YUV420P,
                               PIX_FMT_YUV422P, PIX_FMT_YUV444P,
                               PIX_FMT_YUV410P, PIX_FMT_YUV411P,
                               -1}