combine audio streams from truehd extension

[Brian Dodge]

In mpegtsdemux,  Dolby trueHD extended buffers (pes extension bit) 
should be merged with the not-extended-bit-set buffers into a single 
buffer to push out.   The PES parser properly detects the extension 
field length, and I can pull out the proper stream ID from the extension 
header, but I don't see how this info can be sent back up, so that the 
callback knows that this extension buffer should be merged with the next 
buffer (or previous buffer, I don't know enough to know).