Creates more possibilities for vertical and horizontal skyline functions. (issue 7516048)

[k-ohara5a5a]

good enough


https://codereview.appspot.com/7516048/diff/1/scm/define-grobs.scm
File scm/define-grobs.scm (right):

https://codereview.appspot.com/7516048/diff/1/scm/define-grobs.scm#newcode312
scm/define-grobs.scm:312: (vertical-skylines .
,grob::simple-vertical-skylines-from-y-extent)
I'm confused, because the fall-back for vertical-skylines seems to do
exactly the same thing for Spanners, and it looks like this is a
Spanner.

https://codereview.appspot.com/7516048/diff/1/scm/define-grobs.scm#newcode2237
scm/define-grobs.scm:2237: (horizontal-skylines .
,grob::simple-horizontal-skylines-from-x-extent)
The fall-back would be the same if this Grob were marked cross-staff =
#t   Would it be more smarter to mark this and similar grobs cross-staff
?

https://codereview.appspot.com/7516048/