h261enc: Disallow sliced encoding

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>