app, libgimp*, pdb: new "format" type in the PDB.

We have a bunch of special-casing format passing through the PDB, but
either we were only passing the encoding, or else we were reconstructing
the full format through private intermediate functions. In the
space-invasion world, this is not right. Let's have a proper "format"
type for PDB which does all the relevant data-passing for us, once and
for all!

Note that I am creating a wrapper boxed type GimpBablFormat whose only
goal is to have recognizable GValue since Babl types don't have GType-s.
Moreover I'm not using the GeglParamSpecFormat either, because it just
uses pointers which again are a bit annoying in our various PDB code.
Having a simple boxed arg is better.

libgimpcolor/gimpcolor.c

@@ -753,3 +753,39 @@ gimp_color_get_CIE2000_distance (GeglColor *color1,
 
   return dE00;
 }
+
+
+/*
+ * GIMP_TYPE_BABL_FORMAT
+ */
+
+static const Babl * gimp_babl_object_copy (const Babl *object);
+static void         gimp_babl_object_free (const Babl *object);
+
+G_DEFINE_BOXED_TYPE (GimpBablFormat, gimp_babl_format, (GBoxedCopyFunc) gimp_babl_object_copy, (GBoxedFreeFunc) gimp_babl_object_free)
+
+/**
+ * gimp_babl_object_copy: (skip)
+ * @object: a Babl object.
+ *
+ * Bogus function since [struct@Babl.Object] should just be used as
+ * never-ending pointers.
+ *
+ * Returns: (transfer none): the passed @object.
+ **/
+const Babl *
+gimp_babl_object_copy (const Babl *object)
+{
+  return object;
+}
+
+/**
+ * gimp_babl_object_free: (skip)
+ * @object: a Babl object.
+ *
+ * Bogus function since [struct@Babl.Object] must not be freed.
+ **/
+void
+gimp_babl_object_free (const Babl *object)
+{
+}

libgimpcolor/gimpcolor.def

@@ -1,5 +1,6 @@
 EXPORTS
 	gimp_adaptive_supersample_area
+	gimp_babl_format_get_type
 	gimp_bilinear
 	gimp_bilinear_16
 	gimp_bilinear_32

libgimpcolor/gimpcolor.h

@@ -95,6 +95,33 @@ GParamSpec * gimp_param_spec_color_from_string (const gchar *name,
 
 gboolean     gimp_param_spec_color_has_alpha   (GParamSpec  *pspec);
 
+
+/*
+ * GIMP_TYPE_BABL_FORMAT
+ */
+
+/**
+ * GimpBablFormat:
+ *
+ * This type is simply a wrapper around [struct@Babl.Object] when used as
+ * a color format.
+ *
+ * The only reason of this wrapper is to be able to assign a %GType to
+ * Babl formats, e.g. to have typed `GValue`, which is mostly used
+ * internally by our plug-in protocol.
+ *
+ * There is no reason whatsoever to use this type directly.
+ *
+ * Since: 3.0
+ */
+typedef const Babl *                        GimpBablFormat;
+
+#define GIMP_TYPE_BABL_FORMAT               gimp_babl_format_get_type ()
+#define GIMP_VALUE_HOLDS_BABL_FORMAT(value) (G_TYPE_CHECK_VALUE_TYPE ((value), GIMP_TYPE_BABL_FORMAT))
+
+GType   gimp_babl_format_get_type           (void) G_GNUC_CONST;
+
+
 G_END_DECLS
 
 #endif  /* __GIMP_COLOR_H__ */