app: fix autotools build.

My previous commit broke the autotools build. Apparently when using
g_object_unref(), some C++ symbol leaked into libapppaint.a archive
library, hence the main binaries (e.g. gimp-2.99) could not be linked
without adding -lstdc++ flag:

> /usr/bin/ld: paint/libapppaint.a(gimppaintcore-loops.o): undefined reference to symbol '__gxx_personality_v0@@CXXABI_1.3'
> /usr/bin/ld: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: error adding symbols: DSO missing from command line

Not exactly sure why using this GLib function in particular caused this,
but let's just try another approach in order not to link the main binary
with C++ standard lib.
Instead let's manage all GeglOperation allocation in gimp-layer-modes.c
by adding a gimp_layer_modes_exit() function and some static array for
storing operation object of each layer mode.

app/gegl/gimp-gegl.c

@@ -104,6 +104,7 @@ gimp_gegl_exit (Gimp *gimp)
 {
   g_return_if_fail (GIMP_IS_GIMP (gimp));
 
+  gimp_operations_exit (gimp);
   gimp_parallel_exit (gimp);
 }
 

app/operations/gimp-operations.c

@@ -223,3 +223,11 @@ gimp_operations_init (Gimp *gimp)
   set_settings_folder (GIMP_TYPE_LEVELS_CONFIG,
                        "levels");
 }
+
+void
+gimp_operations_exit (Gimp *gimp)
+{
+  g_return_if_fail (GIMP_IS_GIMP (gimp));
+
+  gimp_layer_modes_exit ();
+}

app/operations/gimp-operations.h

@@ -22,6 +22,7 @@
 
 
 void   gimp_operations_init (Gimp *gimp);
+void   gimp_operations_exit (Gimp *gimp);
 
 
 #endif /* __GIMP_OPERATIONS_H__ */

app/operations/layer-modes/gimp-layer-modes.c

@@ -1110,6 +1110,7 @@ static const GimpLayerMode layer_mode_groups[][2] =
   }
 };
 
+static GeglOperation *ops[G_N_ELEMENTS (layer_mode_infos)] = { 0 };
 
 /*  public functions  */
 
@@ -1124,6 +1125,15 @@ gimp_layer_modes_init (void)
     }
 }
 
+void
+gimp_layer_modes_exit (void)
+{
+  gint i;
+
+  for (i = 0; i < G_N_ELEMENTS (layer_mode_infos); i++)
+    g_clear_object (&ops[i]);
+}
+
 static const GimpLayerModeInfo *
 gimp_layer_mode_info (GimpLayerMode mode)
 {
@@ -1265,55 +1275,54 @@ gimp_layer_mode_get_operation_name (GimpLayerMode mode)
   return info->op_name;
 }
 
+/**
+ * gimp_layer_mode_get_operation:
+ * @mode:
+ *
+ * Returns: a #GeglOperation for @mode which may be reused and must not
+ *          be freed.
+ */
 GeglOperation *
 gimp_layer_mode_get_operation (GimpLayerMode mode)
 {
   const GimpLayerModeInfo *info = gimp_layer_mode_info (mode);
-  GeglNode                *node;
-  GeglOperation           *operation;
+  const gchar             *op_name;
 
-  if (! info)
-    info = layer_mode_infos;
-
-  node = gegl_node_new_child (NULL,
-                              "operation", info->op_name,
-                              NULL);
-
-  operation = gegl_node_get_gegl_operation (node);
-  g_object_ref (operation);
-  g_object_unref (node);
-
-  return operation;
-}
-
-GimpLayerModeFunc
-gimp_layer_mode_get_function (GimpLayerMode mode)
-{
-  const GimpLayerModeInfo  *info = gimp_layer_mode_info (mode);
-  static GimpLayerModeFunc  funcs[G_N_ELEMENTS (layer_mode_infos)];
+  op_name = gimp_layer_mode_get_operation_name (mode);
 
   if (! info)
     info = layer_mode_infos;
 
   mode = info - layer_mode_infos;
 
-  if (! funcs[mode])
+  if (! ops[mode])
     {
       GeglNode      *node;
       GeglOperation *operation;
 
       node = gegl_node_new_child (NULL,
-                                  "operation", info->op_name,
+                                  "operation", op_name,
                                   NULL);
 
       operation = gegl_node_get_gegl_operation (node);
 
-      funcs[mode] = GIMP_OPERATION_LAYER_MODE_GET_CLASS (operation)->process;
-
+      g_object_ref (operation);
       g_object_unref (node);
+
+      ops[mode] = operation;
     }
 
-  return funcs[mode];
+  return ops[mode];
+}
+
+GimpLayerModeFunc
+gimp_layer_mode_get_function (GimpLayerMode mode)
+{
+  GeglOperation *operation;
+
+  operation = gimp_layer_mode_get_operation (mode);
+
+  return GIMP_OPERATION_LAYER_MODE_GET_CLASS (operation)->process;
 }
 
 GimpLayerModeBlendFunc

app/operations/layer-modes/gimp-layer-modes.h

@@ -24,6 +24,7 @@
 
 
 void                       gimp_layer_modes_init                      (void);
+void                       gimp_layer_modes_exit                      (void);
 
 gboolean                   gimp_layer_mode_is_legacy                  (GimpLayerMode           mode);
 

app/paint/gimppaintcore-loops.cc

@@ -1876,13 +1876,12 @@ struct DoLayerBlend : Base
 
   static constexpr gint max_n_iterators = Base::max_n_iterators + 2;
 
-  const Babl                     *iterator_format;
-  GimpOperationLayerMode         *layer_mode = NULL;
-  const GimpPaintCoreLoopsParams *params;
+  const Babl             *iterator_format;
+  GimpOperationLayerMode *layer_mode = NULL;
 
   explicit
   DoLayerBlend (const GimpPaintCoreLoopsParams *params) :
-    Base (params), params(params)
+    Base (params)
   {
     layer_mode = GIMP_OPERATION_LAYER_MODE (gimp_layer_mode_get_operation (params->paint_mode));
     layer_mode->layer_mode      = params->paint_mode;
@@ -1902,18 +1901,6 @@ struct DoLayerBlend : Base
     g_return_if_fail (gimp_temp_buf_get_format (params->paint_buf) == iterator_format);
   }
 
-  DoLayerBlend (const DoLayerBlend &algorithm) :
-    Base (algorithm.params), params(algorithm.params)
-  {
-    layer_mode = GIMP_OPERATION_LAYER_MODE (g_object_ref (algorithm.layer_mode));
-    iterator_format = algorithm.iterator_format;
-  }
-
-  ~DoLayerBlend ()
-  {
-    g_clear_object (&layer_mode);
-  }
-
   template <class Derived>
   struct State : Base::template State<Derived>
   {