[gimp] app: in gimppaintcore-loops, make CanvasBufferIterator self-contained

[Ell <ell src gnome org>]

commit 0d1f7241121e4c027402eb5c9fce5aa6e8cd997e
Author: Ell <ell_se yahoo com>
Date:   Mon Feb 11 09:27:04 2019 -0500

    app: in gimppaintcore-loops, make CanvasBufferIterator self-contained
    
    In gimppaintcore-loops, make the CanvasBufferIterator algorithm
    helper-class self-contained, not having to rely on AlgorithmBase.

 app/paint/gimppaintcore-loops.cc | 31 ++++++++++++++++++++++++-------
 1 file changed, 24 insertions(+), 7 deletions(-)
---
diff --git a/app/paint/gimppaintcore-loops.cc b/app/paint/gimppaintcore-loops.cc
index c313ee4bc6..0a46e7c433 100644
--- a/app/paint/gimppaintcore-loops.cc
+++ b/app/paint/gimppaintcore-loops.cc
@@ -232,16 +232,13 @@ struct AlgorithmBase
    * Algorithms that redefine 'filter' should bitwise-OR their filter with that
    * of their base class.
    */
-  static constexpr guint          filter               = 0;
-
-  /* See CanvasBufferIterator. */
-  static constexpr GeglAccessMode canvas_buffer_access = {};
+  static constexpr guint          filter          = 0;
 
   /* The current maximal number of iterators used by the hierarchy.  Algorithms
    * should redefine 'max_n_iterators' by adding the maximal number of
    * iterators they use to this value.
    */
-  static constexpr gint           max_n_iterators      = 0;
+  static constexpr gint           max_n_iterators = 0;
 
   /* Non-static data members should be initialized in the constructor, and
    * should not be further modified.
@@ -558,14 +555,34 @@ struct DispatchStipple
 
 template <class Base,
           guint Access,
-          guint BaseAccess = Base::canvas_buffer_access>
+          guint BaseAccess>
+struct CanvasBufferIterator;
+
+template <class Base,
+          guint Access,
+          guint BaseAccess>
+static constexpr GeglAccessMode
+canvas_buffer_iterator_access (CanvasBufferIterator<Base, Access, BaseAccess> *algorithm)
+{
+  return CanvasBufferIterator<Base, Access, BaseAccess>::canvas_buffer_access;
+}
+
+static constexpr GeglAccessMode
+canvas_buffer_iterator_access (AlgorithmBase *algorithm)
+{
+  return {};
+}
+
+template <class Base,
+          guint Access,
+          guint BaseAccess = canvas_buffer_iterator_access ((Base *) NULL)>
 struct CanvasBufferIterator : Base
 {
   /* The combined canvas-buffer access mode used by the hierarchy, up to, and
    * including, the current class.
    */
   static constexpr GeglAccessMode canvas_buffer_access =
-    (GeglAccessMode) (Base::canvas_buffer_access | Access);
+    (GeglAccessMode) (BaseAccess | Access);
 
   using Base::Base;
 };