Add missing drums to MIDIsymbols

Just add 'D' glyphs for now.

78 Mute Cuica
79 Open Cuica
80 Mute Triangle
81 Open Triangle
Add GS percussion instruments
gen_adldata: Replace missing instruments with 0
gen_adldata: Avoid conflicts between melodic and drum banks

In some of the bnk files (eg: Int) the percussive and non-percussive
instruments are detected wrongly resulting in the melodic bank
being overwritten by drum instruments. I noticed this while listening
the Descent level 3 music with the accompanying intmelo bank.
Pass the percussion flag into LoadBNK explicitly.
instruments.py: 256-color experiments