[mythtv] GOP_BYFRAME vs. GOP_START and mythcommflag --rebuild

[Bill]

Agreed about the decoders.  Thanks for the clarification.

I've been looking at making a patch, to convert to use only GOP_BYFRAME, but 
don't know what to do with the code

bool is_ivtv = (keyframedist == 15) || (keyframedist ==12);
positionMapType = (is_ivtv) ? MARK_GOP_START : MARK_GOP_BYFRAME;

in HandleGopStart in avformatdecoder.cpp.

I would tend to set keyframedist=1 and positionMapType=MARK_GOP_BYFRAME for 
everything in both HandleGopStart and Openfile, but not sure about the ivtv 
thing.  I believe that these changes would allow mythcommflag to generate a 
good position map.

I have a sneaking suspision that making these changes will help improve 
seeking on mpg's without position maps, since the position map is stored 
incorrectly if keyframdist is greater than 1 and changes during playback and 
when the positionMapType gets set to MARK_GOP_BYFRAME.