Wrong ties when moving Tie_engraver from Voice to Staff

[Helge Hafting]

I have tried moving the Tie_engraver from Voice to Staff, in order to 
typeset music with cross-voice ties.

(The music doesn't really do that, but lilypond need it this way. The 
piece has a cross-staff beam, and all the notes are tied to a 
cross-staff chord that follows. And lilypond can only do cross-staff 
chords with two voices, while the cross-staff beam must be a single
voice. So some notes must then tie from one voice to another, and
moving the Tie_engraver to Staff accomplish this nicely. Moving the 
Tie_performer in a similiar way even achieves correct MIDI for this 
construct.)

Unfortunately, this move also gives me a lot of unwanted ties. When one 
voice starts a tie at some point in time, then every other voice who has 
a note starting at the same time also gets a tie! That is wrong and 
unwanted. It is as if the Tie_engraver merely notices that a tie start 
at some point in time, and don't understand that tieing shouldn't apply 
to *all* notes that start at that point.

The Tie_engraver is already capable of not tieing all notes in a chord, 
so I hope this is fixable.

A minimal 4-note example is attached. Two are tied, the other two are 
not. But lilypond ties all.

Helge Hafting

tiebug2.ly
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