Log Message:
libopenmpt: update to 0.5.5.

### libopenmpt 0.5.5 (2021-01-31)

*  [**New**] `Makefile` `CONFIG=emscripten` now supports
   `EMSCRIPTEN_TARGET=audioworkletprocessor` which builds an ES6 module in
   a single file with reduced dependencies suitable to be used in an
   AudioWorkletProcessor.

*  [**Bug**] stb_vorbis: Fix decoding of Vorbis streams without comments which
   affected most Vorbis samples since stb_vorbis v1.20.

*  `openmpt::ext::interactive::set_pitch_factor` wasn't applied to OPL voices.
*  OPL channel state (in particular current patch) is now updated when seeking.
*  The FT2 tremolo quirk is now also applied to MOD files. FT2 just copied the
   quirky code from ProTracker!
*  DMF: Preserve effects better in some situations where there is more than one
   effect in a pattern cell.
*  DMF: Improve import of finetune effect with parameters larger than +/-15.

*  mpg123: Update to v1.26.4 (2020-12-24).
*  pugixml: Update to v1.11.4 (2020-12-22).