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

[Bill]

Use mythtranscode --buildindex instead.  This will only work for recordings, 
if its a video (under mythvideo), you need the patch to ticket # 3533.

Actually, mythcommflag will sometimes create the MARK_GOP_START  and 
sometimes as MARK_GOP_BYFRAME .  Just depends what the initial key frame 
distance is in the file.

Also if it chooses MARK_GOP_BYFRAME it will almost always create a wrong 
position map since it divides the frame # by the key frame distance.  It 
shouldn't, it should just store the frame # in the position map.  When the 
file is played back with a position map of  MARK_GOP_BYFRAME, the key frame 
distance is set to 1, which is not the same as when mythcommflag generated 
the position map in the first place.