[mythtv] Timebar in playback wrong due to incorrect marks, from mythcommflag --rebuild

[wsslkmpr at nc.rr.com]

> I am using mythtv as a mirror of the files on my tivo. The files get 
> pulled, converted through vsplit from .ty files to .mpg files, and added 
> to the database based on the xml file from the tivo. I then try and run 
> 'mythcommflag --rebuild' on them. myFramesPlayed and totalFrames are 
> both correct in NuppelVideoPlayer::RebuildSeekTable, however the MARK 
> entry placed in the recordedmarkup table is short by around a factor of 
<snip>

I ended up fixing this by forcing keyframedist to 1 in 
AvFormatDecoder::OpenFile (avformatdecoder.cpp)
just like what is done for avi files.