XML and JSON formats may contain only relative path to audio file

[DanielSWolf]

The XML and JSON exporters add metadata to the output file, including the path of the audio file. If the audio file is given as a relative path on the command line, it is also written as a relative path. This means that code interpreting that information may not be able to find the original audio file.

When using the Vegas plugin on such an XML file, it responds with the (not really helpful) message "Media stream not specified."

As a fix, the exporters should always write the absolute file path.