Subtitles: fix missing CEA-708 closed captions.

This is a quick hack version of
7d9bf3193e0f3d1795c1ead6f16aa386ab7de587, which fixes a CEA-708
decoding bug exposed by recent changes to NBC's CEA-708 encoding.
Fix "mythcommflag --rebuild" for non-HDPVR recordings.

By default, only identify IDR keyframes for h.264 recordings.
However, since DVB-S recordings may lack IDR frames, we check after
1000 frames whether any keyframes have been found, and if not, back up
and restart using the more lenient setting.

Adapted from 802e32ba4dc8852da56db170893e95cbc28637e3 and
77bf3275ef3133d81fc979ae40d9728523ff9974.
Fix a memory leak introduced in 92016b949.  Fixes #11329.

Signed-off-by: Jim Stichnoth <jstichnoth@mythtv.org>
Fix transitions from a DVD still frame into video.  Fixes #11347.

Signed-off-by: Jim Stichnoth <jstichnoth@mythtv.org>
(cherry picked from commit fcf176db6e06928364f35f32aa022696fb19edf6)
Temporary fix for #11159: Fix playback of HD-PVR recording

This revert FFmpeg commit bfca35114ae174bbabd88526379d95d4022a98b1
(ref https://ffmpeg.org/trac/ffmpeg/ticket/2062)
(cherry picked from commit d34833f2eeab06bc17338e4d5a8bc33589f41506)

Conflicts:
	mythtv/external/FFmpeg/libavcodec/h264.c
Allow seeking back through the live TV chain.

 Fixes a regression introduced in 0.25.  Refs #11174.