Log Message:
schismtracker: update to 20210525.

Summary of changes since 20200412 release:

- Add support for saving MOD files
- Fix conflict in MIDI setup when ALSA and OSS are both available
- Allow setting program on MIDI drums
- Fix instrument and sample panning overwriting channel panning
- Fix issues related to portamento with Amiga slides
- Support compressed stereo samples in ITI/ITS files
- Convert XM/MOD EA0/EB0 to D00 when loading
- Clamp S3M mixing volume to valid values on save
- Improve autovibrato accuracy
- Fix noteless instrument numbers updating instrument panning
- Fix handling of zero-value linear slides
- Read extended OpenMPT version information
- Fix instrument sample mapping in the IMF loader
- Fix memory access bug in the MTM loader