Add support for AVC3

[mickd5]

AVC3 stream plays audio but with no video. The extra codec data is not out of of band but in the stream so there is no SPS or PPS found. As the codec data maybe only 7 bytes long dvbvideosink will give warning message "codec_data too short(1)" or in some cases the data was 11 bytes but again SPS and PPS are not out of band for AVC3. The warning in this case was "codec_data too short(2)". Changing the code and just writing a 4 byte start code to the buffer when this type of stream is detected seems to work and video is displayed with no issues but not sure if this is the correct approach.

Some sample streams below
http://rdmedia.bbc.co.uk/dash/ondemand/elephants_dream/1/client_manifest-all.mpd
http://rdmedia.bbc.co.uk/dash/ondemand/testcard/1/client_manifest-ctv-events.mpd