[mythtv] 2019-render: nvdec decoding artefacts in the editor

[John Pilkington]

Hi Mark:  I see a new commit that might have fixed this - but I had 
started, so I'll finish...

This is a niche effect but it's still there (0515eacc), even with 
progressive frames.  ffmpeg works fine.  Both work well in normal 
playback but then ffmpeg demands more of the cpus.

Would it be feasible to switch to ffmpeg when displaying a static frame?

Thanks,

John P