More lossless mythtranscode fixes

Lossless (mpeg2) transcodes in master now seem to work perfectly on every file
I've tried included files which have always failed to
transcode. Feedback to the forum, irc, mythtv-dev or mythtv-users list
would be appreciated. I'd like to know whether this resolves long
standing lossless mpeg2 transcoding failures for anyone else. I make no
promise to fix unresolved issues, but it would be good to know if
things have moved in the right direction.

Fixes #12280