whiteout shouldn’t affect other grobs on same layer

[Malte Meyn]

Hi list,

in the following example the NoteHead.whiteout doesn’t only cover the 
tie but also one NoteHead whites out the other:

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\version "2.19.82"

\relative <<
  {
    \override NoteHead.whiteout = 3
    \override NoteHead.layer = -1
    r2 <f' a>
  } \\ {
    \override Tie.layer = -2
    <a d,>1~ q4
  }
>>
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

Of course, it would be possible to use a \tweak here:

    r2 <\tweak whiteout 3 \tweak layer -1 f' a>

But that makes the code less readable and if more notes are affected 
it’s a pain.

IMO it would be nice if a grob’s white box on one layer wouldn’t cover 
grobs on the same layer; or at least not grobs of the same type on the 
same layer.

Would it be possible to put the whiteout part half a layer below so that 
\override NoteHead.layer = -1 puts the notehead at layer -1 and the 
surrounding white box at layer -1.5? (Or, if you want only integers: -1 
puts the notehead at layer -2 and the white box at layer -3.)

Cheers,
Malte