container/mts/encoder.go: change the way meta data is added external to the package

[Saxon Milton]

Currently the MTS package provides a global meta.Meta variable for the addition of metadata to encoded MTS. This method of setting metadata external to the package is not ideal, particularly with the current situation where we get a panic when we don’t initialise this (in the case that we don’t need to set any particular metadata). A preferred option would be to provide a callback function that can be used to set metadata.