Fix up SCI0_LATE variant of the driver so that it actually plays anything (and even correctly). SCI1 hasn't seen any testing from me so far. I don't know whether that version works. Same for SCI0_EARLY...
Many engines follow the advice in audio/midiparser.h and create a
"pseudo-MidiDriver" subclass. But MidiParser really only needs a tiny
subset of the MidiDriver capabilities, namely those found in
MidiDriver_BASE. So we try to subclass from that whenever possible; this
allows us to remove many stub methods, and enables further future
simplifications.