NotePool: Fix obscure legato pitch bug

NotePool: Fix obscure legato pitch bug

Fix obscure bug where the pitch of a previously played and currently
releasing legato note (i.e. a note formed by playing one note, and
holding it while playing another) will jump to the pitch of a newly
played legato note (i.e., again, not a single note, but a first
note played and helt while playing a second), instead of releasing at
its original pitch.

This is due to applyLegato() setting all note descriptors to the same
note number, including those releasing. Although setting previous
notes to the same pitch as new ones will give the impression of a
single voice being played, this did not happen for the first note
played, and furthermore, gathering all released voices at the new
pitch means there will be an unintended plurality of voices at that
pitch.