avcodec/h263dec: use PIX_FMT_GRAY8 when the user specified CODEC_FLAG_GRAY

Fixes Ticket4542
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h263dec.c

@@ -48,6 +48,9 @@ static enum AVPixelFormat h263_get_format(AVCodecContext *avctx)
     if (avctx->codec->id == AV_CODEC_ID_MSS2)
         return AV_PIX_FMT_YUV420P;
 
+    if (CONFIG_GRAY && (avctx->flags & CODEC_FLAG_GRAY))
+        return AV_PIX_FMT_GRAY8;
+
     return avctx->pix_fmt = ff_get_format(avctx, avctx->codec->pix_fmts);
 }