mxfdemux: Fix issues at EOS

The essence track position should only be overriden if we sucesfully switched to another position. In case of EOS we do not want to override it else we would increase the track position again at the end of this function

mxfdemux: Fix issue with re-syncing

In case of re-syncing (i.e. moving to another partition to avoid too much of an interleave), there was previously no checks to figure out whether a given partition was already fully handled (i.e. when coming across it again after a previous resync).

In order to handle this at least for single-track partitions, check whether we have reached the essence track duration, and if so skip the partition.