[mythtv] render2019: step-length oddities in video editing

[John Pilkington]

I use the editor to establish cutpoints in almost all my recordings, 
cutting at keyframes.  In 2019render 33b109b1106e the static images 
shown at keyframes are often blurred, but stepping forward and back by 
keyframes is consistent.

Nominal 1-frame steps forward are OK, and advancing around 7 frames 
usually gives a better image; but steps back are by keyframe intervals. 
Nominal 0.5-second steps do the same, and nominal 1-second steps seem to 
go to keyframes in both directions.  The larger steps aren't exact, but 
near enough.

I've tried this with both h264 and mpeg2 recordings.  I haven't done a 
direct comparison with master, but the feel is different.

John P