GUS instrument mappings bug

The GUS instrument mappings bug is a bug in vanilla Doom related to how music is played back when using a Gravis Ultrasound card.

The bug relates to how the DMXGUS configuration lump is parsed by Doom's music code; MIDI instruments are mistakenly mapped to the wrong instruments. A prominent example of this can be heard in MAP01: Entryway (Doom II) where a dissonant clanging sound replaces a percussion drum instrument that is supposed to be played.

The bug can be worked around by loading a modified DMXGUS lump.