This should fix bug reported in https://forum.zdoom.org/viewtopic.php?
f=104&t=56682.
There were two problems:
1. Algorithm to select a free voice was not appropriate. Changed to a
simple "least-recently-freed voice" algorithm.
2. Sustained voices were treated as used voices and never got replaced.
Now sustained voices are preferentially replaced when there are no free
voices.
src/sound/oplsynth/musicblock.cpp:3:10: fatal error: 'muslib.h' file not found
src/sound/oplsynth/oplio.cpp:410:12: error: use of undeclared identifier 'cos'
src/sound/oplsynth/oplio.cpp:410:41: error: use of undeclared identifier 'sin'