Issue 4907: Midi_walker::do_start_note: skip ignored notes in stop_note_queue

Issue 4907: Midi_walker::do_start_note: skip ignored notes in stop_note_queue

For each semitone pitch value, stop_note_queue is likely supposed to
contain at most one Midi_note event with its "ignore_" flag set to
false, and the comparisons between notes of equal semitone pitch to be
always done between the input note and this unique queued note that is
not (yet) being ignored.

If notes which are already being ignored are not skipped in the loop,
the task of raising the "ignore_" flags for note events of equal
semitone pitch (overlapping in time) which stop before the maximum
stopping time of these notes may, due to breaking out of the loop,
fail to work if the queue grows to contain three or more notes of equal
semitone pitch, leading to the emission of premature "note off" events
for this pitch, as demonstrated, for example, in
<http://lists.gnu.org/archive/html/bug-lilypond/2016-06/msg00042.html>.