Reduce unnecessary non-const MIDI buffer access.
Fix lost MIDI note offs and controllers.

Fix initial read of discrete MIDI controllers.

Fix spurious note offs when starting to play in the middle of a note.

Faster search for initial event when cached iterator is invalid.

So much for dropping the cached iterator.  The iterator is responsible for
handling note offs, so that doesn't work.  This design means we have some stuck
note issues at the source read level, but they should be taken care of by the
state tracker anyway.