[mythtv] fixes/31 branch created!

[Mark Kendall]

I've pushed a few improvements in the last few days for the Pi4 - Pi3
not tested. My Pi4 has gone a little nuts however - so will need to
start from scratch before I can go any further.

With the latest fixes, I'd recommend the following display profile
settings for the Pi4 (probably the same for Pi3):-

- decoder - V4L2
- max cpus - 4
- deinterlacer (both single and double rate) - low quality, Prefer
OpenGL deinterlacers.

This should give the best performance for both H264 hardware decode
and fallback to software decoding (e.g. MPEG2 on Pi4, or no licence on
Pi3).

In terms of performance, there is a definite performance limitation
tying to decode 1080P H264. Frames are dropped.

1080i is in theory fine - no frames are dropped, CPU load is low but
playback has intermittent tearing (depending on the source material).
As far as I can tell, this is a Pi4 firmware/driver issue. There are
plenty of reported issues with the Pi4 and tearing and recognition
from the Pi foundation/engineers that this is an unresolved issue. The
giveaway for me is that testing with 1080i, software decode of MPEG2,
there is intermittent breakup of the displayed frames - but absolutely
no indication from the mythtv logs that anything is wrong. Under load,
the timing signals to the display are erratic.

Regards
Mark