Log Message:
libopenmpt: update to 0.5.3.

ChangeLog:

### libopenmpt 0.5.3 (2020-10-25)

 *  [**Sec**] Possible hang if a MED file claimed to contain 256 songs. (r13704)

 *  [**Bug**] libopenmpt: `openmpt::is_extension_supported2()` exported symbol
    was missing (C++).
 *  [**Bug**] `openmpt::module::set_position_seconds` sometimes behaved as if
    the song end was reached when seeking into a pattern loop and in some other
    corner cases.

 *  Increase threshold for ignoring panning commands from 820 to 830.
 *  Subsong names now fall back to the first pattern's name if empty.
 *  MO3: Avoid certain ModPlug hacks from being fixed up twice, which could lead
    to e.g. very narrow pan swing range for old OpenMPT IT files saved with a
    recent MO3 encoder version.
 *  MO3: Some files with corrupted envelope data could be rejected completely
    (normally libopenmpt should fix up the envelope data).
 *  MO3: Song metadata didn't correctly identify MPTM as source format (it
    appeared as IT instead).
 *  STM: Change tempo computation to behave like Scream Tracker 2.3 instead of
    Scream Tracker 2.2, as the playback frequencies we use for sample playback
    are closer to those of Scream Tracker 2.3.
 *  PLM: Percentage offset (Mxx) was slightly off.
 *  WOW: Fix loading of several files and harden WOW detection.