minimp3 library from https://github.com/lieff/minimp3
commit 50d2aaf360a53653b718fead8e258d654c3a7e41 (2021-11-27)
The following changes have been made:
 *  minimp3.c has been added
 *  some warnings have been fixed
 *  all modifications are marked by /* OpenMPT */
