audio/fasttracker2: Updates to v1.39

Changes since v1.38

v1.39 - 08.11.2020
- Fixed yet another issue where junk could be drawn after the loop end point in
  the sample data's waveform in the sample editor (if zoomed out).
- Fixed an issue with loading XMs with more than 128 instruments (OpenMPT etc).
  This only worked if the extra instruments were empty. Now it will properly
  load the extra instruments and discard them after the loading was complete
  (since we only support up to 128 instruments).
- Small code cleanup

(fox)