improve Master System SMPS loading, misc fixes

- cleaned Fading routine
- fixed PWM note 80h
- rearranged PlayMusic routine (removing duplucate code)
- made it load PSG channels into TRACK_MUS_PSGx tracks when used in
  FMChnList (e.g. Master System SMPS)
add support for preSMPS instruments

- made function for instrument table generation, removing duplicate code
  [loader_smps.c]
- added interleaved instrument definitions (preSMPS-style)
- added Instrument Library format
add "Restore BGM channel after SFX end"
add 2-operator FM drum support (preSMPS drums)