Gets vertical skylines from grob stencils (issue 5626052)

[mtsolo]

Reviewers: ,

Message:
Sorry in advance for the whitespace errors.

This patch provides a generic framework for building vertical skylines
out of boxes by traversing through a stencil.  It currently only
implements skylines for three stencil types (draw-line, named-glyph, and
glyph-string) and can do more for other glyphs.  It is modular, so as
skyline estimation improve for a given stencil, that function can be
worked on alone w/o touching the other parts of stencil-integral.scm.

The implementation for glyph-string is ugly and slightly incorrect for y
heights of these stencils...it would be infinitely easier if the stencil
could contain information about y extents, but unfortunately,
PangoGlyphGeometry does not carry height information.  This error does
not have a bearing on the visual results in this patch, but it'd be good
to find a way to make the dimensions exact.

I'm also not sure that line thickness is tacked on correctly for the
draw-line boxes, but it seems to yield correct results.

Cheers,
MS

Description:
Gets vertical skylines from grob stencils

Please review this at http://codereview.appspot.com/5626052/

Affected files:
  A input/regression/tuplet-bracket-vertical-skylines.ly
  A input/regression/volta-bracket-vertical-skylines.ly
  M lily/axis-group-interface.cc
  A lily/box-scheme.cc
  M lily/include/box.hh
  M lily/include/skyline.hh
  M lily/skyline-pair.cc
  A lily/skyline-scheme.cc
  M lily/skyline.cc
  M lily/stencil-scheme.cc
  M lily/tuplet-bracket.cc
  M lily/volta-bracket.cc
  M scm/define-grobs.scm
  M scm/lily.scm
  A scm/stencil-integral.scm