Change JS MTSDemuxer to work with live data.

MTSDemuxer currently keeps a reference of all the data that it demuxes, this behaviour is unintended and causes an error when used with continuous live data.

It should be changed so that the data it demuxes is demuxed and then returned to the caller without keeping a reference.

A more appropriate name for the function that does this would be “demux” rather than the existing “append”.