libzvbi-teletextdec: output ass subtitles instead of plain text

Signed-off-by: Marton Balint <cus@passwd.hu>
libzvbi-teletextdec: dont ignore memory allocation error silently

Signed-off-by: Marton Balint <cus@passwd.hu>
libzvbi-teletextdec: propagate errors generated in page handler

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