Bug: instrumentName is not being vertically centered when using template 'piano centered dynamics'

[Moritz Baumann]

> I'm not top posting.

Example:
\version "2.11.23"
     upper = \relative c'' {
       \clef treble
       \key c \major
       \time 4/4
     
       a b c d
     }
     
     lower = \relative c {
       \clef bass
       \key c \major
       \time 4/4
     
       a2 c
     }
     
     dynamics = {
       s2\fff\> s4
       s\!\pp
     }
     
     pedal = {
       s2\sustainDown s2\sustainUp
     }
     
     \score {
       \new PianoStaff <<
         \set PianoStaff.instrumentName = "Piano "
         \new Staff = "upper" \upper
         \new Dynamics = "dynamics" \dynamics
         \new Staff = "lower" <<
           \clef bass
           \lower
         >>
         \new Dynamics = "pedal" \pedal
       >>
       \layout {
         \context {
           \type "Engraver_group"
           \name Dynamics
           \alias Voice % So that \cresc works, for example.
           \consists "Output_property_engraver"
     
           \override VerticalAxisGroup #'minimum-Y-extent = #'(-1 . 1)
           \override DynamicLineSpanner #'Y-offset = #0
           pedalSustainStrings = #'("Ped." "*Ped." "*")
           pedalUnaCordaStrings = #'("una corda" "" "tre corde")
     
           \consists "Piano_pedal_engraver"
           \consists "Script_engraver"
           \consists "Dynamic_engraver"
           \consists "Text_engraver"
     
           \override TextScript #'font-size = #2
           \override TextScript #'font-shape = #'italic
     
           \consists "Skip_event_swallow_translator"
     
           \consists "Axis_group_engraver"
         }
         \context {
           \PianoStaff
           \accepts Dynamics
         }
       }
     }
     \score {
       \new PianoStaff <<
         \new Staff = "upper" << \upper \dynamics >>
         \new Staff = "lower" << \lower \dynamics >>
         \new Dynamics = "pedal" \pedal
       >>
       \midi {
         \context {
           \type "Performer_group"
           \name Dynamics
           \consists "Piano_pedal_performer"
         }
         \context {
           \PianoStaff
           \accepts Dynamics
         }
       }
     }