[mythtv] Picture in Picture broken

[Mark Kendall]

So I've pushed a number of updates to master - nothing in fixes/31 just yet.

In summary:-

- PiP should be working
- only the main player will use hardware decoding - all PiPs will use
software decoding (and this is enforced when swapping PiPs - so it
will always be the main player that uses hardware decoding).
- PiPs have a new sync method that should ensure PiP plays at correct
rate - regardless of the main player's frame rate.
- there is still a remote possibility of locking up the frontend when
using hardware decoding. One of the newer updates attempts to
catch/prevent decoder deadlocks but it simply cannot catch all cases.
That said in the testing I've done over the last few days, I've failed
to trigger a deadlock again. A complete fix requires some significant
surgery to the decoder code - the use of the global, static
avcodeclock mutex is the problem.

Regards
Mark