Log Message:
libopenmpt: update to 0.4.12.

ChangeLog:

### libopenmpt 0.4.12 (2020-04-13)

 *  IT: Vibrato was too fast in Old Effects mode since libopenmpt 0.3.
 *  XM: Treat 8bitbubsy's FT2 clone exactly like Fasttracker 2 with respect to
    compatibility and playback flags. For example, FT2 Pan Law was not applied.
 *  DMF: Some files had a wrong tempo since libopenmpt 0.2.5705-beta15.