libxmp output plugin

[ghost]

Another one for the low-priority request department. :)

As compared to modplug, xmp has more accurate playback, supports a wider range of formats, and seems to be more frequently updated:

http://sourceforge.net/projects/xmp/files/libxmp/4.3.8/

[velusip]

I would vouch for libxmp as well. Many of the features are inspired by the BASS library which can't be beat for module playback. While BASS is free for non-commercial use, the non-GPL license only makes some details of redistribution and forking complicated, so libxmp is the best thing for this context.

[tidux]

Bumping this from beyond the grave for a reason. The refusal to implement layered plugins in #923 means that all the tens of thousands of modules shipped as zip/lharc on modarchive, aminet, etc. can't be opened with the modplug plugin without doing major filesystem surgery - and because most of those archives also contain multiple files it'd be a manual process. libxmp handles archive parsing internal to itself, which would avoid the issue for tracker modules where this gap is most keenly felt.