Add support for text/ass input from AV_CODEC_ID_DVB_TELETEXT

[kasper93]

dvb_teletext subtitles/pages decoded by lavc are only supported in bitmap form.

Currently setting --sub-lavc-o=txt_format=1 (text) or --sub-lavc-o=txt_format=2 (ass) causes:

sub/lavc: unsupported subtitle type from libavcodec

It is hard-coded here, that AV_CODEC_ID_DVB_TELETEXT will produce bitmaps. https://github.com/mpv-player/mpv/blob/09606b9db9e228ac80cadb5fd7875ad60c9bd178/sub/sd_lavc.c#L82

Expected behavior of the wanted feature

dvb_teletext can be rendered as text or ass.

TODO:

• remove AV_CODEC_ID_DVB_TELETEXT assumption
• move teletext page selection to common code
• fix other issues

Sample

sub.zip