#5135 PecentRepeat with small 'slope causes collisions

[Colin Campbell]

As reported by Harm:

the following code gives bad output (collisions) for PecentRepeat with
small 'slope

\repeat percent 2 { c'4 d' e' f' \override PercentRepeat #'slope = #0.2 }

This happens in all tested versions from 2.19.60 back to 2.12.3

My first thought was the stencil extents are not calculated correctly
though the code below seems to proof this wrong:

boxPercentRepeat =
  \once \override PercentRepeat #'after-line-breaking =
    #(lambda (grob)
      (ly:grob-set-property! grob 'stencil
        (stencil-with-color
          (box-stencil
            (ly:grob-property grob 'stencil)
            0.2 0.5)
          red)
          ))

slopePercentRepeat =
  \once \override PercentRepeat #'slope = #0.2

\repeat percent 2 {
  c'4 d' e' f'
  \boxPercentRepeat
  \slopePercentRepeat
}