Configure the h.264 parser to identify only IDR keyframes.

This makes keyframes identified by "mythcommflag --rebuild" and
mythtranscode much closer to reality, though there are still
discrepancies compared to the recorder and raw ffmpeg.

mythtv/libs/libmythtv/avformatdecoder.cpp

@@ -354,6 +354,8 @@ AvFormatDecoder::AvFormatDecoder(MythPlayer *parent,
 
     cc608_build_parity_table(cc608_parity_table);
 
+    m_h264_parser->use_I_forKeyframes(false);
+
     LOG(VB_PLAYBACK, LOG_DEBUG, LOC + QString("PlayerFlags: 0x%1")
         .arg(playerFlags, 0, 16));
 }