h261enc: Disallow sliced encoding

This avoids trying to do sliced encoding, even if a slice/packet
size is requested (via the -ps option or the rtp_payload_size
field), since the encoder currently doesn't support it (or at least
our decoder can't decode it, even if the h261_encode_gob_header
function is hooked up to be called from the slicing part in
mpegvideo_enc.c).
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/mpegvideo_enc.c

@@ -590,6 +590,7 @@ av_cold int ff_mpv_encode_init(AVCodecContext *avctx)
         s->out_format = FMT_H261;
         avctx->delay  = 0;
         s->low_delay  = 1;
+        s->rtp_mode   = 0; /* Sliced encoding not supported */
         break;
     case AV_CODEC_ID_H263:
         if (!CONFIG_H263_ENCODER)