markLengthOn pushes notes with ledger lines

[Saul Tobin]

{
  \markLengthOn
  \tempo "Tempo longer longer" 4=120
  c''''1
  \mark\default
}

While floating to the side of the musical material could be appropriate
behavior for rehearsal numbers, MetronomeMarks should always float above
the music. They should not affect horizontal spacing of the music except to
stretch the length of entire measures.