AvFormatDecoder: Reset the video codec on stream changes.

I've been staring blankly at this trying to figure out why it hasn't
caused problems before - only to realise that live tv changes always
force a full teardown of the decoder and the codec never changes for dvd
playback.

Should, and appears to, fix a number of blu ray playback issues when the
bd library switches to a title with a different codec.

mythtv/libs/libmythtv/avformatdecoder.cpp

@@ -1793,8 +1793,12 @@ int AvFormatDecoder::ScanStreams(bool novideo)
                     }
                 }
 
-                bool handled = false;
-                if (!using_null_videoout && mpeg_version(enc->codec_id))
+                video_codec_id = kCodec_NONE;
+                int version = mpeg_version(enc->codec_id);
+                if (version)
+                    video_codec_id = (MythCodecID)(kCodec_MPEG1 + version - 1);
+
+                if (!using_null_videoout && version)
                 {
 #if defined(USING_VDPAU)
                     // HACK -- begin
@@ -1816,7 +1820,6 @@ int AvFormatDecoder::ScanStreams(bool novideo)
                     {
                         enc->codec_id = (CodecID) myth2av_codecid(vdpau_mcid);
                         video_codec_id = vdpau_mcid;
-                        handled = true;
                     }
 #endif // USING_VDPAU
 #ifdef USING_VAAPI
@@ -1830,7 +1833,6 @@ int AvFormatDecoder::ScanStreams(bool novideo)
                     {
                         enc->codec_id = (CodecID)myth2av_codecid(vaapi_mcid);
                         video_codec_id = vaapi_mcid;
-                        handled = true;
                         if (!no_hardware_decoders &&
                             codec_is_vaapi(video_codec_id))
                         {
@@ -1849,7 +1851,6 @@ int AvFormatDecoder::ScanStreams(bool novideo)
                     {
                         enc->codec_id = (CodecID)myth2av_codecid(dxva2_mcid);
                         video_codec_id = dxva2_mcid;
-                        handled = true;
                         if (!no_hardware_decoders &&
                             codec_is_dxva2(video_codec_id))
                         {
@@ -1859,12 +1860,12 @@ int AvFormatDecoder::ScanStreams(bool novideo)
 #endif // USING_DXVA2
                 }
 
-                if (!handled)
+                // default to mpeg2
+                if (video_codec_id == kCodec_NONE)
                 {
-                    if (CODEC_IS_H264(enc->codec_id))
-                        video_codec_id = kCodec_H264;
-                    else
-                        video_codec_id = kCodec_MPEG2; // default to MPEG2
+                    VERBOSE(VB_IMPORTANT, LOC +
+                            "Unknown video codec - defaulting to MPEG2");
+                    video_codec_id = kCodec_MPEG2;
                 }
 
                 if (enc->codec)