Log Message:
libopenmpt: update to 0.5.12.

### libopenmpt 0.5.12 (2021-10-04)

 *  [**Sec**] Possible crash when loading malformed MDL files. (r15603)

 *  [**Bug**] Fixed various undefined behaviour found with ubsan.

 *  Seeking with sample sync sometimes didn't compute the correct sample
    position with pingpong-looped samples.
 *  IT: Tremor command I11 erroneously behaved like I00 (use previous
    parameter) unless IT Old Effects were enabled.
 *  PTM: Panning was translated wrong in some edge cases.
 *  IMF / PTM: Note Slide commands were sometimes slightly off.
 *  OKT: Better support for fine note slides.
 *  DBM: Echo enable effect parameter range checks were incorrect.
 *  XM: Sample texts in XMs made with MadTracker are now also decoded using
    Windows-1252 encoding.

 *  in_openmpt: Song metadata is no longer reverted when viewing file info.