Playback: Improved triggers for low bitrate optimisations.

The optimisations added in d207a0b are being triggered
unnecessarily. This is due to a combination of different initial
thresholds in that commit and lowering the acceptable bitrate in
a1022c6.

The fix is twofold:-

- adjust the initial bitrate thresholds to ensure any optimisations are
only triggerred for streams with a bitrate below 500Kb/s.
- any video codec scanned in AvFormatDecoder that has a zero bitrate is
forced to 500Kb/s. This is an extension of the previous hack for H.264
streams.

Overall, this ensures that the previous behaviour (initial fill size of
32KB, rising as needed) is preserved for anything with a video stream
and generally in the vast majority of cases.

Fixes recent playback of avi files and, amongst others, BB HD (both
detected as zero bitrate).

Refs #9824.

mythtv/libs/libmythtv/avformatdecoder.cpp

@@ -1768,14 +1768,14 @@ int AvFormatDecoder::ScanStreams(bool novideo)
 
                 codec_is_mpeg = CODEC_IS_FFMPEG_MPEG(enc->codec_id);
 
+                // ffmpeg does not return a bitrate for several codecs and
+                // formats. Forcing it to 500000 ensures the ringbuffer does not
+                // use optimisations for low bitrate (audio and data) streams.
                 if (enc->bit_rate == 0)
-                    unknownbitrate = true;
-
-                // HACK -- begin
-                // ffmpeg is unable to compute H.264 bitrates in mpegts?
-                if (CODEC_IS_H264(enc->codec_id) && enc->bit_rate == 0)
+                {
                     enc->bit_rate = 500000;
-                // HACK -- end
+                    unknownbitrate = true;
+                }
 
                 StreamInfo si(i, 0, 0, 0, 0);
                 tracks[kTrackTypeVideo].push_back(si);

mythtv/libs/libmythtv/ringbuffer.cpp

@@ -352,7 +352,6 @@ void RingBuffer::CalcReadAheadThresh(void)
     const uint KB2   =   2*1024;
     const uint KB4   =   4*1024;
     const uint KB8   =   8*1024;
-    const uint KB16  =  16*1024;
     const uint KB32  =  32*1024;
     const uint KB64  =  64*1024;
     const uint KB128 = 128*1024;
@@ -366,8 +365,7 @@ void RingBuffer::CalcReadAheadThresh(void)
                      (estbitrate >  9000) ? KB256 :
                      (estbitrate >  5000) ? KB128 :
                      (estbitrate >  2500) ? KB64  :
-                     (estbitrate >  1000) ? KB32  :
-                     (estbitrate >   500) ? KB16  :
+                     (estbitrate >=  500) ? KB32  :
                      (estbitrate >   250) ? KB8   :
                      (estbitrate >   125) ? KB4   : KB2;
     if (rbs < CHUNK)