script-column: earlier scripts support later scripts; issue 3683 (issue 35010043)

[dak]

https://codereview.appspot.com/35010043/diff/1/lily/script-column.cc
File lily/script-column.cc (right):

https://codereview.appspot.com/35010043/diff/1/lily/script-column.cc#newcode156
lily/script-column.cc:156: use all the scripts so far as support for the
current grob
A question of understanding: supports are not transitive automatically
(if a supports b, and b supports c, a supports c)?

Because if they are, it should be sufficient to only add all of the
grobs since the last grob with explicit outside_staff_priority.

https://codereview.appspot.com/35010043/diff/1/lily/script-column.cc#newcode159
lily/script-column.cc:159: for (SCM t = ss; scm_is_pair (t) &&
!scm_is_eq (t, s); t = scm_cdr (t))
Drop the scm_is_pair (t) condition.  It can never be false, and makes
the loop more confusing to read (the condition looks like the loop skips
over a->a pairings but with cover both a->b and b->a when it doesn't).

https://codereview.appspot.com/35010043/