Gets vertical skylines from grob stencils

lily/line-spanner.cc

 /*
   This file is part of LilyPond, the GNU music typesetter.
 
   Copyright (C) 2000--2012 Jan Nieuwenhuizen <janneke@gnu.org>
 
   LilyPond is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
   (at your option) any later version.
 
@@ skipping 338 matching lines @@
     }
 
   for (LEFT_and_RIGHT (d))
     {
       if (stencils[d])
         span_points[d] += dz_dir *
                           (stencils[d]->extent (X_AXIS)[-d] / dz_dir[X_AXIS]);
     }
 
   Offset adjust = dz.direction () * Staff_symbol_referencer::staff_space (me);
-
   Offset line_left = span_points[LEFT] + (arrows[LEFT] ? adjust * 1.4 : Offset (0, 0));
   Offset line_right = span_points[RIGHT] - (arrows[RIGHT] ? adjust * 0.55 : Offset (0, 0));
+
   if (line_right[X_AXIS] > line_left[X_AXIS])
     {
       line.add_stencil (Line_interface::line (me, line_left, line_right));
 
       line.add_stencil (Line_interface::arrows (me,
                                                 span_points[LEFT],
                                                 span_points[RIGHT],
                                                 arrows[LEFT],
                                                 arrows[RIGHT]));
     }
@@ skipping 14 matching lines @@
                "bound-details "
                "extra-dy "
                "gap "
                "left-bound-info "
                "note-columns "
                "right-bound-info "
                "simple-Y "
                "thickness "
                "to-barline "
               );