libzvbi-teletextdec: split dvb packet to slices

Instead of using the demux function of libzvbi to split the packet to slices
(vbi lines), lets do it ourselves.

- eliminates the 1 frame delay between page input and output
- handles non-ascending line numbers more gracefully
- enables us to return error codes on some invalid packets instead of silently
  ignoring them

Signed-off-by: Marton Balint <cus@passwd.hu>
