Log Message:
libopenmpt: update to 0.5.1.

ChangeLog:

### libopenmpt 0.5.1 (2020-07-26)

 *  [**Bug**] `libopenmpt/libopenmpt.h` failed to compile with
    `LIBOPENMPT_NO_DEPRECATE` defined.

 *  MPTM: Qxy now retriggers OPL notes if new compatibility flag is set in file.
 *  MPTM: Bring back old OPL note end-of-envelope behaviour for files made with
    OpenMPT 1.28.
 *  IT: Global volume slides with both nibbles set preferred the "slide up"
    nibble over the "slide down" nibble in old OpenMPT versions, \ 
unlike other
    slides. Such old files are now imported correctly again.
 *  IT: Fixed an edge case where, if the filter hit full cutoff / no resonance
    on the first tick of a row where a new delayed note would be triggered, the
    filter would be disabled even though it should stay active. Fixes trace.it
    by maddie.
 *  OXM: Some sample loops were not imported correctly.
 *  XM: Out-of-range arpeggio clamping behaviour broke in OpenMPT 1.23.05.00.
    The arpeggios in Binary World by Dakota now play correctly again.
 *  S3M: Support old-style sample pre-amp value in very early S3M files.
 *  S3M: Only force-enable fast slides for files ST 3.00. Previously, any S3M
    file made with an ST3 version older than 3.20 enabled them.
 *  S3M: Only apply volume and middle-C speed on instrument change if the new
    sample slot has sample data.
 *  MOD: Fix an infinite loop in GamerMan by MrGamer by playing non-ProTracker
    MODs more like FT2 would.
 *  M15: Improve tracker detection heuristics to never assume SoundTracker 2.0
    if there is a huge number of Dxx commands, as that is a definite hint that
    they should be treated as volume slides. Fixes Monty On The Run by
    Master Blaster.
 *  MO3: Support OPL patches in MO3 files created from MPTM and S3M.
 *  DBM: If a global pattern command would be lost because both effect commands
    in a cell would have to go into the regular effect column (e.g. a speed and
    a tempo command), the lost command is now attempted to be written into a
    different cell on the same row. Fixes "Party-Question V" by grogon.

 *  mpg123: Update to v1.26.3 (2020-07-16).
 *  stb_vorbis: Update v1.20 commit b42009b3b9d4ca35bc703f5310eedc74f584be58
    (2020-07-13).