XM player bugs and missing features

[electronoora]

• [x] something causing sample sequences to play off sync in te-2rx.xm, little_man.xm and yuki_satellites.xm (was incorrect envelope loop length)
• [x] panning sounds kind of wrong but not always
• [ ] noise/crackle just before a song ends.
• [ ] occasional clicks (sounds like broken interpolation over sample loop point?)
• [ ] some volume column effect commands are unimplemented
• [ ] some FT2-specific effect commands are unimplemented
• [ ] instrument vibrato is unimplemented
• [ ] compatibility for versions other than 0x0104 is uncertain