polyphony and TabStaff: don’t put two voices on the same string

[Malte Meyn]

Hi list,

this was reported at the LilyPond facebook group. When having “real” 
polyphony with multiple Voice contexts, the notes are put on the strings 
independently which can lead to several voices with different frets on 
the same string.

The first three “chords” on the TabStaff should look the same (1st 
string 3rd fret, 2nd string 5th fret). I’m not sure about the correct 
handling of unisons (second three “chords”) in polyphony because I don’t 
play guitar ;)

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\version "2.21.0"

music = \relative {
  <g' e>2
  << g e >>
  << \tweak TabNoteHead.extra-offset #'(0.3 . 0.3) g \\ e >>
  <e e>
  << e e >>
  << \tweak TabNoteHead.extra-offset #'(0.3 . 0.3) e \\ e >>
}

<<
  \new Staff { \clef "treble_8" \music }
  \new TabStaff \music
>>
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

I tweaked the extra-offsets so that you can see that LilyPond just 
prints two TabNoteHeads in the exact same space.

Cheers,
Malte