wrong horizontal trill position in small staff lines

[Werner LEMBERG]

\version "2.13.1"

\header { texidoc = "
The horizontal position of the beginning of a trill spanner must be
positioned correctly relative to the note head it is attached to, even if
scaled to a smaller size.

Here is a counterexample, clearly showing an unwanted horizontal shift.
This is a regression; it has worked with version 2.11.13, for example.
" }

<<
  \new Staff \with {
    fontSize = #-6
    \override StaffSymbol #'staff-space = #(magstep -6)
  }
  \relative {
    c'1\startTrillSpan | c\stopTrillSpan |
  }

  \new Staff \relative {
    c'1\startTrillSpan | c\stopTrillSpan |
  }
>>

% EOF