[gegl] buffer: rewrite headers to avoid inline function ptr declaration

[Øyvind Kolås <ok src gnome org>]

commit 77b2ffc6c4580ab301a27f8596c729c8285329fe
Author: Øyvind Kolås <pippin gimp org>
Date:   Tue Feb 20 21:26:03 2018 +0100

    buffer: rewrite headers to avoid inline function ptr declaration
    
    Based on patch by Souichi Takashige in bug #793659

 gegl/buffer/gegl-tile-source.h |   13 +++++++------
 1 files changed, 7 insertions(+), 6 deletions(-)
---
diff --git a/gegl/buffer/gegl-tile-source.h b/gegl/buffer/gegl-tile-source.h
index cc78fd6..dced599 100644
--- a/gegl/buffer/gegl-tile-source.h
+++ b/gegl/buffer/gegl-tile-source.h
@@ -39,17 +39,18 @@ G_BEGIN_DECLS
 #define GEGL_TILE_SOURCE_GET_CLASS(obj)  (G_TYPE_INSTANCE_GET_CLASS ((obj),  GEGL_TYPE_TILE_SOURCE, 
GeglTileSourceClass))
 
 typedef struct _GeglTileSourceClass GeglTileSourceClass;
-
-struct _GeglTileSource
-{
-  GObject   parent_instance;
-  gpointer  (*command)  (GeglTileSource  *gegl_tile_source,
+typedef gpointer  (*GeglTileSourceCommand)  (GeglTileSource  *gegl_tile_source,
                          GeglTileCommand  command,
                          gint             x,
                          gint             y,
                          gint             z,
                          gpointer         data);
-  gpointer  padding[4];
+
+struct _GeglTileSource
+{
+  GObject               parent_instance;
+  GeglTileSourceCommand command;
+  gpointer              padding[4];
 };
 
 struct _GeglTileSourceClass