sd_lavc: enable teletext

Whitelisting supported codecs is (probably) still better than just
allowing everything, given the weird FFmpeg API. I'm also assuming
Libav doesn't even have the codec ID, but I didn't check.

Also add a --teletext-page option, since otherwise it decodes every
teletext page and shows them in succession.

And yes, we can't use av_opt_set_int() - instead we have to set it as
string. Because FFmpeg's option system is terrible.