fix #35661: bad breath position if no tick tag

This is a proposed fix.  I don't know what is safe to assume about the different ways breath marks might be encountered, but I tested that this code handles the ways I have actually seen:

1) <breath> tag precededed and followed by explicit <tick> tag (this is how 2.0 normally writes)

2) <breath> tag contains a <tick> tag (this is how 1.3 normally writes)

3) no <tick> tag present; marking is assumed to apply to previous chord (this is how 1.3 "sometimes" writes)

My code won't work if the breath tag is ever intended to apply to the *next* chord, but I've never seen this.