[mythtv] Raspberry Pi playback smoothness patches

[Doug Larrick]

Greetings MythTV developers,

Some of you may remember me from a really long time ago. I was involved
in the 2004-ish timeframe in the initial addition of US ATSC HDTV
support to MythTV. Most of that code is gone, but my copyright remains
in the vsync.cpp/vsync.h source files. I've been a continuous user of
MythTV since that time, and about a year ago I switched my primary
frontend to a Raspberry Pi, thanks to the excellent efforts of Peter
Bennett and company.

I have been doing some work recently to improve the smoothness of
playback on Raspberry Pi. I have two small patches which I have
prepared for your consideration. 

The first restores the "realtime thread priority" feature which has
gone missing over the years (despite the settings item still being
present), allowing video synchronization to be scheduled as a real-time 
thread.  

I've made a PR and Trac ticket for this and cross-linked them: https://
code.mythtv.org/trac/ticket/13170#ticket

The second increases the default number of OMX buffers used for
decoding. Testing on my systems shows that with the current setting,
the decoder occasionally runs out of buffers when decoding some high-
bitrate over-the-air ATSC 1080i programming. Increasing this to 15
buffers completely smooths out these scenes, and fits within 256MB of
Pi GPU memory. Recognizing this setting might not work with some RPi
setups with less memory devoted to GPU, I also added some code to retry
with fewer buffers until they fit.

This one is https://code.mythtv.org/trac/ticket/13171#ticket

Note: I've made these PRs against master, but my testing has been
against fixes/29.

-Doug
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