Use a proper struct for envelope properties
Add a config option to specify the extra MIDI volume scaling

The value specified is in decibels.
Update MIDI config option texts
Rename aftertouch to keypressure
Better handle original key and overriding root key
Partially handle the irom sub-chunk

Reject soundfonts that have ROM sample types without a valid irom sub-chunk.
Fix a couple warning messages to show the correct instrument ID
Put the version and irom storage into the Soundfont struct
Remove redundancy in a couple error messages
Check for 0-sized sf2 structure arrays