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.

DOCS/man/options.rst

@@ -1795,6 +1795,10 @@ Subtitles
     of subtitles across seeks, so after a seek libass can't eliminate subtitle
     packets with the same ReadOrder as earlier packets.
 
+``--teletext-page=<1-999>``
+    This works for ``dvb_teletext`` subtitle streams, and if FFmpeg has been
+    compiled with support for it.
+
 Window
 ------