Do not allow non-content stream types to extend the format context's duration as this
affects the UI and therefore seek capabilities.

DVB subtitle/teletext streams are private streams (stream type 0xBD) and may have their
own timebase or PTS values that are widely different than video stream's,
or not set properly. Regardless of any handling of such cases in FFmpeg, XBMC shouldn't
extend play duration unless an actual content stream has passed it's "end".
Merge pull request #4337 from voguemaster/only_allow_extend_duration_real_streams

Do not allow non-content stream types to extend the format context's dur...