mpegtsmux - support the MPEG-2 AAC variant
mpegtsmux supports muxing AAC, using the semantics from the MPEG-4 specs that extend the AAC that was defined in MPEG-2. mpegtsmux had incorrect caps for the latter case - treating audio/mpeg,mpegversion=2 caps as meaning MPEG-2 part 3 audio, instead of AAC which is MPEG-2 part 7. Fix that, and add a variant of the raw->ADTS header code that works for the original MPEG-2 AAC specs