staff-staff padding all appears after the last nonstaff

[Keith E OHara]

Fellow bug-chasers,
  In the new rod-and-spring vertical spacing system, rods sometimes appear in 
surprising places.

  If we have two staves on either side of a non-staff, such as Dynamics or 
Lyrics, the new system ensures that the requested padding between staves 
appears as a clearance below the last of the non-staff lines.   Also, any 
staff-staff minimum-distance in excess of that required to avoid collisions, is 
placed below the lowest intervening non-staff.

  The intention, I think, was to let whichever requires the most room -- 
intervening non-staffs, padding between notes on the staves themselves, or 
minimum-distance between staves -- determine the staff spacing.  Then the 
non-staff lines should be spaced between the staves according to the options 
such as UP/DOWN/CENTER that we chose for them.

  The override in this example makes the issue more obvious.  With no overrides 
we notice only that the centered dynamics are not quite centered, but a little 
high because the default staff-staff padding of 1 staff-space is placed below 
the dynamics.
-Keith

--8<--
\version "2.13.39"
\new PianoStaff \with {
   \override StaffGrouper #'staff-staff-spacing #'minimum-distance = #12
} <<
    {g' b b g'}
    \new Dynamics { s\< s\> s s\!}
    {d'' b'' b'' d''} >>