Allows inner-markup spacing using skylines. (issue 13341044)

[dak]

Where's the point?  We don't use that kind of arrangement in our code,
and it's not efficiently doable anyway.

A conceivable application would be for determining systems spacing
manually (for example, generating sequences of TeX \vspace and \break
for LilyPond-book that will reserve normal rectangular space at page
breaks, but interleaving staff spacing when not broken), but this
function is mostly useless for that purpose.  One can only indirectly
try to guess the size of the spacing, one can't reuse a skyline pair(!)
when one wants to compare against several neighbors (requiring _two_
calculation of each skyline pair when assembling a vertical
arrangement), and the spacing is appears to be determined differently
than the vertical spacing engine does it usually.

And for emulating vertical spacing, the stack-stencil operation would be
more important anyway.  But what is really wanted is a primitive just
giving out distance, and calling conventions that avoid a double
calculation of each skyline pair when stacking things.


https://codereview.appspot.com/13341044/diff/1/lily/stencil.cc
File lily/stencil.cc (right):

https://codereview.appspot.com/13341044/diff/1/lily/stencil.cc#newcode271
lily/stencil.cc:271: add_at_edge (a, d, s, padding, false);
That's nonsensical.  C++ knows defaults arguments for functions.

https://codereview.appspot.com/13341044/diff/1/lily/stencil.cc#newcode313
lily/stencil.cc:313: offset = (*first_skyp)[d].distance
((*second_skyp)[-d]);
This is an expensive non-cached operation, and yet this calculates
skyline _pairs_ and ignores half of them.

https://codereview.appspot.com/13341044/diff/1/scm/define-markup-commands.scm
File scm/define-markup-commands.scm (right):

https://codereview.appspot.com/13341044/diff/1/scm/define-markup-commands.scm#newcode2038
scm/define-markup-commands.scm:2038: (ly:stencil-combine-at-edge m1 axis
dir m2 0.0 #t)))
Why use the same function in the first place when the last argument is
never used as a variable?

https://codereview.appspot.com/13341044/