Bad extent-estimate with \RemoveEmptyStaffContext

[Nicolas Sceaux]

Hi,

Extent estimate computation seems not to take account of not
displayed staves.
In the following example, the second system has
 (-17.00, -0.19) Y-extent
 (-41.30, -0.19) extent-estimate

\version "2.13.2"
\paper { annotate-spacing = ##t }
\score {
  <<
    \new StaffGroup <<
      \new Staff { c'1 \break R1 }
      \new Staff { c'1 \break R1 }
      \new Staff { c'1 \break R1 }
    >>
    \new Staff { c'1 c' }
    \new Staff { c'1 c' }
  >>
  \layout {
    \context { \RemoveEmptyStaffContext }
  }
}