[mythtv] MythCodecContext: 2 concurrent hardware frames contexts

[John Pilkington]

The playback problem that I saw earlier was with 32.Pre.3.ga83c0d

A few minute's viewing with 32.Pre.8.g46f840 and the nVidia 440.59 
driver suggests that the problem is fixed.  I still see some loss of 
picture rigidity during panning, but only DLNA (or the android leanback 
frontend on guaranteed non-interlaced h264 content) have done it better.

I would say it's ready to push to fixes.  Thanks.

John