Uses only unpure-pure containers to articulate unpure-pure relationships (issue 3199)

Previously, LilyPond used several different lists in define-grobs.scm
to define relationships between unpure and pure functions.  This patch
eliminates these lists, using unpure-pure containers to articulate
these relationships.

The modifications required to implement this change are described below:

-) axis-group-interface.cc
A Scheme function is no longer needed to determine pure relevant grobs.
All grobs can have their Y-extents meaningfully pure-evaluated now. The
worst-case scenario is that they evaluate to false. Dead grobs, on the
other hand, are never pure relevant. The calls to is_live are the only
holdovers from the old pure-relevant? scheme function.

-) grob-closure.cc
We allow unpure-pure containers in simple closures.

-) grob-property.cc
call_pure_function no longer looks up a Scheme module. Because there are
no hard-coded lists in Scheme any more, the function is smaller and
writing it in C++ gets slight efficiency gains.

-) grob.cc
pure_stencil_height used to be a Scheme function in define-grobs.scm.
Because it is much simpler (it no longer makes references to lists defined
in Scheme), it can be implemented in C++.

-) pure-from-neighbor-engraver.cc
Similar to axis-group-interface.cc, the pure-relevant distinction is
no longer important.

-) side-position-interface.cc
In order to avoid issues with alterBroken, we only check pure properties
before line breaking.

-) simple-closure.cc
Simple closures were incorrectly evaluated when containing unpure-pure
containers. This rectifies that.

-) stencil-integral.cc
Several pure equivalent functions needed to be written for skylines.

-) define-grobs.scm
Multiple overrides must be changed to unpure-pure containers. Previous
hard-coded lists are all deleted and several functions moved to C++ (see
above).

-) output-lib.scm
Several common unpure-pure containers used in define-grobs.scm are
defined here. Several functions from define-grobs.scm pertaining to
grob offsets are moved to this file.

1 files changed, 13 insertions, 4 deletions

diff --git a/lily/separation-item.cc b/lily/separation-item.cc
index 6ff2928072..8a32363eb4 100644
--- a/lily/separation-item.cc
+++ b/lily/separation-item.cc
@@ -150,10 +150,19 @@ Separation_item::boxes (Grob *me, Grob *left)
       Interval extra_height = robust_scm2interval (elts[i]->get_property ("extra-spacing-height"),
                                                    Interval (0.0, 0.0));
 
-      x[LEFT] += extra_width[LEFT];
-      x[RIGHT] += extra_width[RIGHT];
-      y[DOWN] += extra_height[DOWN];
-      y[UP] += extra_height[UP];
+      // The conventional empty extent is (+inf.0 . -inf.0)
+      //  but (-inf.0 . +inf.0) is used as extra-spacing-height
+      //  on items that must not overlap other note-columns.
+      // If these two uses of inf combine, leave the empty extent.
+
+      if (!isinf (x[LEFT]))
+        x[LEFT] += extra_width[LEFT];
+      if (!isinf (x[RIGHT]))
+        x[RIGHT] += extra_width[RIGHT];
+      if (!isinf (y[DOWN]))
+        y[DOWN] += extra_height[DOWN];
+      if (!isinf (y[UP]))
+        y[UP] += extra_height[UP];
 
       if (!x.is_empty () && !y.is_empty ())
         out.push_back (Box (x, y));