Allows glissandi to be continuous at line breaks (issue4273098)

[mtsolo]

Reviewers: ,

Message:
I'm using this is in a piece I'm working on and figured it may be useful
in LilyPond.  Let me know what you think!

Description:
Allows glissandi to be continuous at line breaks

Please review this at http://codereview.appspot.com/4273098/

Affected files:
  M lily/line-spanner.cc
  M scm/define-grobs.scm


Index: lily/line-spanner.cc
diff --git a/lily/line-spanner.cc b/lily/line-spanner.cc
index  
082db2f4b6b85b228b4a8055b9397d3900798ff9..5936342d09a849b1c3308a124be8dfb4d34ec96d  
100644
--- a/lily/line-spanner.cc
+++ b/lily/line-spanner.cc
@@ -283,6 +283,12 @@ Line_spanner::print (SCM smob)
       return SCM_EOL;
     }

+  Interval normalized_endpoints = robust_scm2interval (me->get_property  
("normalized-endpoints"), Interval (0, 1));
+  Real y_length = span_points[RIGHT][Y_AXIS] - span_points[LEFT][Y_AXIS];
+
+  span_points[LEFT][Y_AXIS] += normalized_endpoints[LEFT] * y_length;
+  span_points[RIGHT][Y_AXIS] -= (1 - normalized_endpoints[RIGHT]) *  
y_length;
+
   Stencil line;
   do
     {
Index: scm/define-grobs.scm
diff --git a/scm/define-grobs.scm b/scm/define-grobs.scm
index  
db65c46cf8333fc8c27213af569e7c183b98d6ec..47991a7a3ec4161d3754ac921cbf71cc9b61cc02  
100644
--- a/scm/define-grobs.scm
+++ b/scm/define-grobs.scm
@@ -927,6 +927,7 @@
                          ))
        (gap . 0.5)
        (left-bound-info . ,ly:line-spanner::calc-left-bound-info)
+       (normalized-endpoints . ,ly:spanner::calc-normalized-endpoints)
        (right-bound-info . ,ly:line-spanner::calc-right-bound-info)
        (stencil . ,ly:line-spanner::print)
        (style . line)