[mythtv] Ticket #4872: Detect if video is letterboxed and switch to corect fill mode

[mythtv at longhome.co.uk]

I've noticed that 2.39:1 letterboxed in a 16:9 stream also gets zoomed,
meaning part of the broadcast picture is lost. I would have thought that
only content in a 4:3 stream should be zoomed? Right?