\textLengthOn fails to allow text to align

[Trevor Daniels]

The predefined command \textLengthOn no longer always permits
TextScripts to align.  I suspect it broke when the new skylining code
was implemented, but I haven't verified this.

\textLengthOn is defined to be

  \override TextScript.extra-spacing-width = #'(0 . 0)
  \override TextScript.extra-spacing-height = #'(-inf.0 . +inf.0)

A workaround is to change extra-spacing-width to #'(0 . 0.2),
but this seems a bit arbitrary.  So I'm not sure if the bug is in the
skylining code or whether \textLengthOn just needs to be adjusted.

The example in the snippet Vertically aligned dynamics and textscripts
as it appears in the NR in section
http://www.lilypond.org/doc/v2.17/Documentation/notation/expressive-marks-attached-to-notes#dynamics
unfortunately now demonstrates this fault.

Mike: could you please comment?
 
Here's an example:

music = \relative c' {
  c2^\markup { \huge gorgeous } c^\markup { \huge fantastic }
}

\score {
{
  \music
  \break
  \textLengthOn
  \override TextScript.Y-extent = #'(-1.5 . 1.5)
  \override TextScript.staff-padding = #0.1
  \music
  \textLengthOff
  \break
  \override TextScript.Y-extent = #'(-1.5 . 1.5)
  \override TextScript.staff-padding = #0.1
  \override TextScript.extra-spacing-width = #'(0 . 0.2)
  \override TextScript.extra-spacing-height = #'(-inf.0 . +inf.0)
  \music
}
\layout {
  ragged-right = ##t
  indent = 0
}
}