mts: Clips should start with a PAT which indicates discontinuities

Each MTS clip generated should start with a PAT immediately followed by a PMT. This should be true both for clips sent over the network, and clips written to a file.

Furthermore, discontinuities will be indicated in the PAT, the PMT, and the immediately following media packet(s) and nowhere else in the MTS stream,. i.e., not mid clip.

This will greatly simplify discontinuity detection by programs that consume our MTS streams.