performance issue with consecutive multi-key chords

[tomcucinotta]

PianoTutor unable to follow user in some relatively quicker pieces with prolonged segments of consecutive multi-key chords. Likely related to how often we flush LEDs onto Arduino, as every key hit by the user used to trigger the MIDI handler in MuseScore, which tells Arduino to turn off the corresponding key, and to flush, to reflect it immediately. However, if this happens multiple times consecutively as part of a single chord being pressed by the user, then we have a problem. This problem was addressed in 1f282c938, but it doesn't seem solved AFAICR.