tsdemux: Make jp2k handling more robust and efficient

tsdemux: Make jp2k handling more robust and efficient

* Avoid copying the pending data and instead create a buffer directly from
  that data with the appropriate offset.
* Locate the jp2k magic to determine the exact location of the (first) frame
  data instead of assuming that the header is of an expected size

https://bugzilla.gnome.org/show_bug.cgi?id=786111